While I failed to find either confirmation or denial to this fact in 15 minutes I devoted to it (just "According to Stardock, the AI in the game doesn't cheat, it plays the same game the player does" from Gamespy), I just think that letting AI in HoMM (sorry, MM:H) see the whole map would be OK for most players. Especially taking into account that players will, in most cases, know the map very well, and you'll use the most basic fog of war (correct?).@Dalai: as far as I know the AI in GalCiv2, while certainly better than most games out there, still cheats a lot - for instance it can see the whole universe (huge advantage to find good planets).
The problem with cheats is they do not help AI to make intelligent decisions, and even to look like it makes intelligent decisions. Pumping up resources, money and army doesn't help to alleviate this problem, it just helps player get more experience and advance heroes faster. Which, in turn, makes things even worse.
As a side note, we were asked about letting AI see the whole map in HoMM-IV many times in hope that it improves it's ability to make good decisions.