TT Tires , Sitting on What?

I have four different trailers that sit on various surfaces....concrete, grass, dirt, gravel. I have not seen any evidence of any difference in any of them over the years, other than the ones on dirt are.....dirty, where they touch the ground. I dunno though, maybe there is "something" going on with some of them that I just can't see.
I don't worry about it. They are all LT tires, I make sure they are inflated correctly.....hook up and go.