I let the FBO fill my plane full service in my hangar and I figure the premium I'm paying for that service is good enough. I don't tip them. I think it's about fourty cents a gallon more so for 60 or 70 gallons I'm 'tipping' an average of ~$20 bucks for not doing it myself.
Anything above and beyond fuel like help with the plane or bags or a ride or anything other than fuel then the tips start.
I worked hard all my life and figure now that I have a couple of nickels to rub together I'll pay or tip to get out of work. I hate fueling the wagon myself. It's a PITA.