The line guys are probably making half what you do. Although in a perfect world, the pax should tip the pilot say, an extra 10 bucks an hour then the plot should chip some of that in to the line guys depending on how many services you required on the ground. If you ask for lav, fuel, gpu, potable water, windshiled cleaning, and gold cap cabin cleaning service, I think at least $20 is in order for the line crew. If you just ask for 2 or 3 of those, $10 would be good enough, 1 out of 3 and a tip really isn't necessary, unless of course it was lav service, in which case $5 is usually a reasonable tip.
That's just my take on it.......
$10/hr tip for the crew
$5 line crew tip for a lav
$10 line crew tip for a lav + 2 other services
$20 line crew tip for a lav and more than 2 other services