I think Mongo is right. For 'odd' corners, or better yet for all corners, especially the floor corners, consider coping them. You'll still have the 'gaps' behind the trim but don't worry - If ANYONE is looking at them and bringing them to your attention, just tell them that they have WAAAAAY too much free time! (Tell them I said that!) Believe me, it's just trim and most likely the VAST majority of corners and walls are 'obstructed'. I'd concentrate on making sure that wherever 2 pieces meet in the MIDDLE of a wall look 'good'. (Here, you cut the 2 'meeting points' at 45 degree angles, and 'join' them.)
As an aside, it's always best to use scrap wood test your ability at mitering crown molding, or any molding for that matter. You needn't necessarily use 'good wood' to test anything. You might even be able to use whatever you removed for testing. BUT, sometimes, you have to use GOOD scrap to test your 'plan'. Yes, I agree, you want to read up about mitering crown molding before you start. It's a bit different then 'normal' molding.