Doc,
I'm relatively new to tradeskilling and all I really do is Implants with my NT. Given that, I usually advertise that I'm putting them together. I used to advertise that I'd take tips, but like you said, that just felt greedy.
If someone offers to tip on the other hand... I'll accept . And to be honest, I've yet to have someone run off with out handing me something.
Still, your point about pricing in advance is a good one. But it sort of takes the fun out of doing a good deed and just helping someone out.