Any risks to over-tightening rear fenders?

[old cool]

Seems like an elementary question, but I occasionally hear a popping noise in rear when cornering. I pulled on driver's side rear fender (fenderside PU) and did get some movement. Passenger side was snug.
Can I just reach under and crank the nuts down or is there another procedure to follow? I don't want to bend or stretch any metals. Seems obvious, but I don't want to over-tighten, strip, or, bend something and have regrets later.