Couple points...
Just signed up for AvPoints yesterday, as a matter of fact, after seeing the placards for years, and the Signature girl at PDK was adamant that it applies equally to avgas/jet-a. If you can grab one of the pamphlets it says the same thing (I just checked) so don't be afraid to call the program # and complain about the specific FBO.
I fly wearing a tie now cuz the pax expect it and DEFINITELY get better treatment than I did wearing dirty, greasy blue jeans but I found on the freight side that a lot of it comes down to how you treat the people. If you actually talk to the line guys and counter chicks, and treat them like THEY are doing a job worthy of respect, than they usually tend to return that style of treatment. It absolutely amazes me the way that a lot of pilots, from PP student wannabes thru freight dawgs thru corporate high poobahs treat people in FBOs---they will almost always respond in kind to the way THEY get treated. My opinion, and as a caveat I HAVE complained to FBO mgrs about bad treatment---if you haven't done anything to warrant bad treatment then don't be afraid to make waves. I had an FBO mgr tell me that he'd call my boss and complain about me---I told him to go ahead, I was an employee earning my keep and he was a guy making $$$ off our fuel bill so who would they believe more? He shut up, and things improved from then on...
Not on the freight destination side, but I don't think I've ever seen a worse FBO than the Gatlinburg/Pigeon Forge airport (GKT). They are rude, unfriendly, unhelpful and just plain nasty! When I said something, their reply---"We're a tourist destination so people will come no matter how we treat them." Unbelievable!