On my vehicles I always highlight. I'm a shadow grey highlighter, followed by very small
corner highlights of fortress grey. I just pick out the edges.
I cheated a little and I didn't bother to highlight the black on all my Tau infantry as if the paint is slightly satin (as GW's generally are) then light will pretty much do the work for you. Also, unless you're painting them for display where close inspection is likely, IMO, it's not necessary to highlight too much because with 6mm infantry on the tabletop, you really can't see black highlights unless they're stark contrast. If you really have to highlight, as Ghudra says, less can be more and maybe just pick out areas with large edges?
As mentioned, if you go with light greys your blacks will look grey unless you wash them with a black after to tone the grey down. You also need to be very careful in applying edge highlights to infantry or the highlights will be "blobby". Be careful drybrushing too as you can cover the black too much - as it's generally a very broad, randomly applied brush stroke - and they will again look grey - you'll most likely need to go back and paint in the flat areas if you dry brush, to bring the original colour back out.
Here's an example of one of my vehicles at least. It's not dead-accurate highlighting as you can see, but that's down to my not-so accurate hands and laziness to clean it up ÂÂ
CMON Hammerhead