What you are supposed to do is cut out the wheel wells and all holes for body posts and stuff before you paint it so you know they are lined up properly. I usually do not finish cutting out the body until im done painting though, I find it a little bit easier to work with when its not entirely cut out. Plus it give you a place to hold onto the body when you paint it, since sometimes you have to pic it up to get paint in certain areas (if your using a spray paint).