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.