I've kept thinking about this feature on and off, especially now with territory effects discussions, and especially for the case in which a tick forests should slow down cavalry. I think the best map on which to theorically apply (were it non abandoned) this feature would be Jurassic, where something like this is partially reproduced by just having the dinosaurs receiving movement bonuses upon starting on favourable terrains (an incomplete solution).
I still tend to think that charging upon exiting would be the best (and also consistent with how fuel cost currently works), but I can see pros and cons, for a number of scenarios. The main example I can think of for charging upon exiting is this case:
You have a "forests_deep" terrain that it is supposed to be bad for cavalry.
This terrain gives combat maluses to cavalry and slows it down (I really don't care about blitz).
You have three possible situations, when you are on the offensive (on the defensive it matters little, as you will have defensive combat maluses for statically defending in the forests):
Your cavalry moves through a deep forest.
Your cavalry starts outside a deep forest and enters a deep forest.
Your cavalry starts inside a deep forest and moves outside a deep forest.
In the first case, it doesn't matter if we charge the malus upon entering or exiting.
In the second case, if we enter the deep forest to do battle, the cavalry already has all the combat maluses, so the terrain issue is already felt, even if it doesn't immediately impact on movement.
In the third case, if the movement malus is given upon entering, being in a deep forest makes absolutely no impact, as you are not going to make combat in it nor be slowed down for exiting it. I don't think anyone would place its strategic reserves in the worst possible terrain for them to move, but with this system that would be not even a problem at all.
So I think charging upon exiting would be preferable to assure the deep forest will have an impact both in the second and the third case. In the second case, it will have an impact because I will have combat maluses (albeit no movement maluses) for invading a deep forest. In the third case, it will have an impact because I will have movement maluses (albeit no combat maluses) for launching an invasion from a deep forest.
Of course, one could give maluses for exiting territory effects by putting a unit in there, giving such maluses if you start in it, but, then, it would be very punitive if coupled with additional costs upon entering. In the example, if a deep forest would give -1 and you would have to pay an additional 1 for entering a deep forest, a cavalry that moves 2 would be completely unable to move from a deep forest into another deep forest.
As long as the binary matter of charging on entering/exiting, I would actually definitely suggest having a property for setting that. This is also the fairest way to settle the matter, leaving the mapmakers the ability to decide whether entering or exiting is the best for what they are doing. If, after some years, it would look like everyone is using one of the two options, and no one the other one, the property can always be removed (and the behaviour changed to the chosen one, if it was not the default) with no issues at all (you just need to run a search to verify who has set that property one way or the other).
I tend to think that a poll about it would probably end up in the entering option winning, though, just because it is the most intuitive of the two. So, I'm not surprised that a number of other games go that way, as well.