Same thing, say, volunteering in a charity shop (which I haven't done for a few years, not for reasons related to this). If I were to be offered pay, it would be too little for me to care about getting, and again I would, sadly, care that people would see me as the person whose job is in a charity shop, rather than the person who is volunteering his time for a good cause.
It's a pretty petty way to think, and I don't always let the "but people will think..." win my internal argument. but it's usually there, and fairly often does sway my decision. (For the record, I don't care that people see me in a positive light, I'd be just as happy donating blood in such a way that no-one ever noticed me doing it, the bit I care about is when I feel I might be seen in a way that I don't like. Still pretty egotistical, but not quite as bad as wanting to show off, I think/hope.)