im sure there are more technical answers, but i like my body to be either touching or almost touching the bumper. not enough to flex the body when you put the body on the car, but enough that it doesnt move a lot when you hit somthing. seems to save the front wheel arch's.
though shoegoo and fiberglass cloth helps that too.
some people mount their bodies quite loosely on the car, but i found that to destroy the reams.