I do try to think of something positive to say about the posts, or I say nothing at all. Also, if the song or genre is a type that I don't understand or appreciate I usually won't comment.
To tell the truth if someone is off key or way out of time, I just don't want to take a chance hurting their feelings or making them mad. After all, it's only my opinion and while I'm almost always right, there is a small, mathematical chance that I could be wrong...NAAH!
It's a good topic and good question though. I hope we get some better answers than mine!
DonM