In my experience, people very rarely use [sic] when quoting on internet forums - readers will assume any quote was copied and pasted; and the quoted text is directly above yours.
>>michae+(OP)
Sometimes people edit their comments after they realize their mistakes; grammar, spelling or otherwise. I use [sic] to denote that "it was like that when I copied it". It's probably from my "old internet" days (/., local forums, etc.).