I think you also need to factor in the level of experience of the people involved. Had I been you, I might not have even blinked an eye. But I'm not you. I have zero experience here and I don't know what's ok and what isn't because there wasn't any guide anywhere (and I expect future volunteers won't necessarily read this post). To me, it can't hurt to leave a comment in all cases, and take the time to explain if you think it may be needed. Better some communication than no communication at all. And, yeah, it's just polite. :)