Maybe it is not intended as such, but a tip is a bribe.
Say I went to the popular ski resort, was in a bit of a hurry so not knowing what was considered a good tip I overtip, to ensure prompt service.
The bar has now been raised for everyone.
I pay for good, prompt service. I tip for above and beyond, but I do not bribe.
IMHO the fuel queue is not decided by the line personnel, it is decided by the order people are there. Air ambulance will always get my place if they are in a rush. Regular customers should not be put ahead of me becasue they tip well and regularly. That FBO will never get a chance to have me as a regular customer if that is how the fuel line is determined.