My view is that you play the game for fun, so in SP mode there is no such thing as cheating - if you think it is ok, then it is ok. Then again, as you will see on this thread, one person's cheesey tactic is another person's good advice. A classic example is training your generals by deliberately fommenting rebellions and crushing them. Some say this is cheating; others say it is what you should do.

Personally I think the AI makes more errors more often than it "cheats". It never seems to build elite armies, and often gets stuck in loops of invasion and withdrawl, building up bad vices for its generals. The difficult thing is to make the game more challenging. I impose limits on myself as follows, playing in GA mode:

never attack an ally
never get excommunicated
always have an objective in any "war" and stop attacking once the objective is achieved
never have an army bigger than one stack (except I do bring a siege train of 4 artillery units and some cheap, fast units like gallowglasses or highlanders) unless it is a crusade.