@Ginger
I agree with the fact that a piece of terrain hasn't got the same "density" all over and that you can get a camouflage effect when standing before any piece of terrain.
But I disagree or better feel different about what the physical piece of terrain represents.
For me it represents not really a piece of terrain, but an area of effect of this kind of terrain. If there are 3 trees on a round base, it represents a forrest with some light outer reaches with more scrub than trees and an area of dense forrest in the middle and probably some clear area with a block house and I don't know what. And it doesn't matter. The piece of terrain shows the area in which my units benefit or are hazarded (is there such a word ?) by this terrain.
When you leave this area the last bit of protection is gone - so no save. The touching and getting a -1 for it, would represent nicely the "hiding by standing in front" aspect you mentioned.

Nonetheless I belong to the group of people who want to have an optical representation of circumstances in their games - and I find this an very important aspect of any game.
A game has to feel and look right to be enjoyable. For close combat units have to touch and not be 20 cm apart. When a unit is routed it should move away from the enemy and not towards him and stare him in the eyes. A unit should show it's combat strength by removing casualties as models instead of tracking the number of losses. In a Napoleonic game at least the standard formations like line, column and square should be recognizable - and a unit should benefit from or be penalized for terrain only when it is visibly in it.

And that is why I prefer the center rule, because it gives the best representation. I would also prefer it (rules wise) for barrages, only when the center of a unit is under the barrage the unit is hit. But as the rules say other wise, I find it a good idea to be consistent and use the same guidance for terrain as for barrages.

Regarding infantry touching vehicles. This is a special case which has it's own strong imagery. When the infantry touches the vehicle you know this represents infantry advancing under the protection of the vehicle, and you know you have no other way to represent this. While with terrain, you can put the center of the base in the terrain or not.

Hmm, I don't think I have heard others mention the "centre rule" as such; but I think we all agree that to gain 'cover' a significant amount of the unit should be actually inside the terrain.

This debate does have another related question; when a transport picks up infantry from their formation under 1.7.5, they have to move into base contact with the infantry. So, if the infantry is deemed to be 'in cover' (however that is defined), does the vehicle have to 'enter' the cover to pick up the troops?

In the FAQ, the answer to this question is that it depends on whether the vehicle has had to enter the terrain; and that depends on the same definition of being 'in' the terrain. I suspect that most people would suggest that the vehicle would be subject to dangerous terrain rule if it was 1mm (or 1/2 a tank track) into the terrain. If this definition is true for vehicles, it ought to be the same for infantry (which is the ERC position IIRC).

Neal is also arguing the same case for 'concealment'; that for consistency, the rule regarding touching vehicles should equally apply to touching terrain. However, it seems that a significant number of people disagree on this (certainly in E-UK circles).

FWIW I play 'in' terrain but by any part of the base, abstractions and all that. Often, some terrain pieces aren't convenient to put figures on them (buildings usually), so for those we play touching.

And like Gavin I also have experience of standing in front of opaque things and not be seen. Like, you know, doors and other people and curtains. Amazing world, isn't it?

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The centre rule is:
To be under a template or in terrain, the unit's center has to be under or in. It's virtually the same as the "half of the unit" has to be under or in rule. The difference is mathematically. The center has no size, so you can put a unit on a road through a forrest, even if the road is smaller than half a stand. Of course this makes only sense, if in you imagery the soldiers are able to move freely about. In a Napoleonic game you might find the "half unit rule" more appropriate, because it prevents stands in Line formation to zoom along roads.

IMHO most gamers think of terrain as areas of effect as I said before. For one it is "natural" to think that if you have an distinctive area that this area is where the effect is. When playing soccer you define your playing field "up to" where the lawn ends and the walkway starts. And the second reason it's just common practice with war games.

While I agree on not be seen despite being in full sight, because of some backdrop it just works the other way round. You just might be seen because of that backdrop. You might have the most advanced camouflage scheme ever, but stand out as soon as you move in front of a uniformly painted house. Also we are not necessarily talking about a sniper hiding in the open, but more often moving or even shooting bodies of men.

But what feels mostly wrong about the touching thing is that you extend your formations ZOC and units weapon range by moving the units as far out as possible (putting the small side of a 40x12mm base in contact with terrain), claiming being in cover, while purposely moving the models completely out of cover optically. It's like having the cake and eat it. IMHO - if you want cover stay in it.


PS.
Writing this, I realize that I'm not even really negative about the idea of touching terrain granting the -1, but it really is the touching with the small side or even an corner, that makes me cringe.
Being concealed when touching with a long edge as completely as possible (i.e. not with the corner) or even better touching with the center of a long edge feels ok for me.
I would even extend that to the vehicle rule - because the image is troops milling about the vehicle and not forming a screen in front of the vehicle as players often do (me included).

While I think that using the center of a unit for being in terrain/under template and touching terrain/vehicles with the center of long edge for claiming -1 would be a very good solution, I understand that it will not not be really considered by most as a possible way (because of how they play now), so feel free to regard this just as the philosophical musings of an old man.

That's pretty much what we do. If you can put it in the terrain, just put it part way in the terrain. If the terrain feature causes physical issues, then just touch.