I will tell you the problems we have at our bodyshop,obviously you can try the fit of the bumper before paint but we often have problems after they have been painted with grilles and trims not fitting for instance and its hard to not only make sure it fits to the car ok before paint (ie bumper to wing gaps,headlamp gaps etc) without having to clip every grille in for instance to make sure they fit then to have to take them all back out again so you can paint it to then have to fit them all again with obvious risks or breaking lugs etc from parts.
We have this issue all the time now with non-gen parts as the insurance company's dont understand that cars with the big mouth grilles which most are now are 50% bumper 50% grilles and trims so the bumper shell has no strength what so ever its when you try to start fitting parts in it then getting it to line up is where the issues normally start
