I've always played it as 1 d6 roll for terrain checks because of the following (highlighted the relevant parts)
Quote:
1.8.1 Dangerous Terrain Test
Roll a D6 when you enter dangerous terrain, or when you start
to move if already in dangerous terrain. On a roll of 1, the unit
is destroyed with no save allowed, but the formation it is part
of does not receive a Blast marker.
when coupled with this ( from section 3.0 War Engines).
Quote:
War engines include all very large machines, such as Ork
Gargants, Imperial Titans, Imperial Guard Baneblades and
Space Marine Thunderhawk Gunships. Because of their huge
size and awesome toughness, war engines are very different to
other units, and because of this many of the core game rules
are modified when it comes to war engines.
That said, the main difference between a war engine and a
normal vehicle is that the war engine has a damage capacity
(abbreviated DC) that shows how many ‘kills’ the war engine
can absorb before it is destroyed. For example, a Warlord Titan
has a DC of 8, and it therefore takes eight ‘kills’ to destroy it.
For most rules purposes, a war engine counts as being ‘worth’
a number of units equal to its starting damage capacity, so a
war engine’s starting damage capacity is also used to work out
how many dice it rolls in an assault, how easy it is to pin and
so on. This and all of the other modifications to the core game
rules are described in detail as follows.
As non of the modifications in this section or following sections mention rolling 1 d6 per DC then I take it as we stick with the core rule of 1 d6 when entering or start moving when in terrain.