If you're just passing through in a Cherokee I wouldn't expect you to tip. As the level and
Quality of service rises so should the need for a tip. Examples:
1. A ride to a near by hotel/ lunch= 2-5$
2. Towing aircraft across a towered field= (if rental=0) if your aircraft= 5$
3. The sky is raining fractional jets, and based charter activity and your left main "needs" 2psi= 5$
4. Bird strike has covered the underside of your airplane with fatty tissue, blood and feathers and you want it washed.= 10$ (Some arse-hole in an Aztec stiffed me on this one!)
5. If you need help with your passenger's baggage or rental car.= 1-2$
6. Bad weather and the fbo hangar full but, line service finds a spot for your bird with a based operator, and a room for you to boot.= 5$
7. Just fuel/oil.= 0$
8. Multiple items needed on last minute departure, (gpu, preheat, fuel, de-ice, rental car return, help with baggage,ect.)= 2-5$
9. Grand Daddy of all requests... Vommit management >20$
Basically, any time you sense that line guy is hustling to get ten things done for five different aircraft and he keeps you on the top of the stack, pop'em a few fazools. If the guy is a slouch, then there is no need to tip. In the line service world tips should be reserved for professional help.
Oh yeah, thanks for using the term "Line help", the term "Line Boy" is demeaning.