Dan I understand your question & concern. Had the very same one last year.
I re-plated m y front bumper not long ago, and while I had your concern I did NOT find it necessary to install any "gaskets."
What I did do however was install each item {bumper, guards, ends, bolts, lic. plate holder etc.} very loosely. then I VERY slowly tightened each one a little bit. As I did this I made sure the items were aligned well, straight etc. And I did NOT crank down really hard. I just snugged up each bolt.
One thing I did notice was that the bumper guards {L & R} did fit "better" on one side than the other. {?}
I did use a "rubber" washer between my "Y" bumper brackets and the inside of my bumper, Just to ensure a good snugger fit. I got the washers from my local hardware store. And they do NOT show up when you look at my front bumper area..so no points lost.
Good luck,
Michael41