Is this the done thing in America, or just because you're interested in aviation?
...Some people do, some don't. In addition to fueling, they help out with bags, will drag out an air compressor if I need one, and let me bum a smoke...I don't mind tipping for that.
I used to work as a line/dispatch person at a small general aviation airport. Did it only one semester because I found out we got no discount on airplane rentals so I bailed for Domino's - more money which I desperately needed for my flight training.
We were never tipped but this was a small airport with few "high rollers". Although I did open up the door for Reba McEntire getting out of a Slo..., I mean Citation once - of course at the time I had no idea who she was. LOL I like country music (but I like almost all music) however she's not my cup of tea. Her pilot left $20 bucks for the two of us who worked that day - much appreciated and that bird got a "free" wash out of us later. ;)
Either way, as lineguys we made $9 an hour and this was some 15 years ago.
PS. Watch that smokin' on airport property - fines are getting steep in the US... :)
Overall, I agree with your ideas on tipping. I'm biased of course but my perception is that the waiters are much more attentive in the US than many other countries. Either way, if in doubt on whether tipping is the norm in the specific profession/country - I tip. YMMV