it depends.
if i saw a band i really hated... let's say good riddance, even though i don't know why i would be near them, i would not walk up to them and tell them that they're greasy jerks. why would they care what i had to say?
it's art, not everyone is going to like it. if someone random came up to me and said my work sucks, it'd probably really upset me or i wouldn't care.
so i wouldn't tell them anything.
now, if they asked me what i thought about their music then i would tell them.