Historically, the defender has certain advantages, if he is prepared for the attack. This is especially true in the case of fortresses/cities, which IMO are to easy to storm in RTW. However, even in the field a strong defensive position was an oft-decisive advantage. You'll note that, throughout history, a good way of 'attacking' an army has been to occupy a strong position commanding its lines of supply, thus forcing them to attack you on a terrain of your chosing. As Foot pointed out, in RTW it can be hard to keep your formation while advancing, especially if the terrain is uneven, although often it's due to the game's pathing fuck-ups , and the AI is simply unable to keep a proper formation at all. Well, historically armiss actually had similar problems maintaining formation while attacking, especially if a clever defender tried to disrupt it (and that's where Hannibal's staggered deployment of his centre at Cannae was pure brilliance).

Personally, I usually let the enemy come to me, unless I've got a very cavalry-heavy army. I usually only move if I need to grab some high ground or consolidate my position against a numerically superior enemy--the exception is when the I'm vastly outnumbered by several armies coming from different directions, in which case I move to eliminate them separately before they can join up, or die trying.