Attention to detail..

Should the detail of the terrain really matter in a hero seige style map? I've put time into the main area the players will be roaming about on, but the rest?

You decide, it is your map.

Good point!

I'd say that you should put as much detail as you can be bothered putting in.

Basically as long as it's unintrusive it will add to the map. On the otherhand i wouldn't lose sleep if you don't have the time to do it.

I'd say put as much detail as how fun your map isn't.
In other words, ayou only need the terrain to be beautiful if player spend a lot of time out of action.
A little hills and doodads are always nice though.

Be careful with detail, if you find yourself getting a cluttered feeling when playing the map, you've gone too far. KISS is the rule to apply to any playable map, Keep It Simple Stupid. Do not pour large amounts of detail into an area simply because you can. You're better off with open, crisp, clean terrain than you are with enclosed, cluttered terrain.