I didn't use spacers either although I was careful about how I tightened the bolts up. On my car the front bumper makes no interference with the paintwork so all I needed to do was set it on squarely and tighten up slowly. I think I had the front grille removed at the time which made access for the nuts easier. (as an aside, I now have the front grille on swivel mounts at the bottom and cable tied at the top, which means it is very easily laid flat for access to the front undertray bolts)
The rear bumper meets the bodywork at either end, I don't know if this is just my car or them all but it means I need to be careful fitting it. To be honest the interference is minimal and you would need to look very closely but there are marks where it was first fitted due to the cellulose paint being soft. You may have better fortune with modern paints but on my last respray I left the outer bolts slack until the paint had fully hardened. The bumper wasn't going to move or vibrate but the bolts were only just past finger tight.
Brian
