A very good reminder!
Two things that always make be shudder are when I hear a diver tell me he is going to take a tank inspection course so he can make some extra money inspecting his buddies' tanks, and when I hear a diver say he thinks he will buy a compressor, and pay for it by doing fills.
However filling buddies' tanks is often pretty much unavoidable, but if one does it's important not to charge for it because that could make it a business, and, in addition to vastly increased liability, bring down OSHA, state and city inspectors, and a host of other things you do not want to know about.
Gratuities - voluntary offerings - are a different matter, but they are best accepted after the fill.