My feeling is that by saying "sorry if this is insulting" or "I apologize in advance" or "before you downvote me" you are rolling over on your belly and appearing vulnerable by acknowledging your vulnerability and trying to show some humility. And in a sense you are setting up the contrast principle because the belly in comparison to the crime just doesn't seem so bad.
whether its effective seems situational
"sorry if this is insulting" won't be effective if what you follow it with is truly insulting
we all know thin skinned people though, sometimes the best way to get them to hear a benign (or neutral, or even constructive critism) comment that they might dismiss is to procede it with "sorry if this is insulting"
so there are certain scenarios where I'd say those phrases are ok (this board with their sometimes quick downvoting, seems like an ok use to me)
putting a sentence about how your comment may be downvoted, is just a request not to have that small number reflexsively downvote, because sometimes that happens
which is maybe better than letting trolls take over, I'm kind of agnostic about it
but alternative viewpoints are often dismissed here
(only example I can readily think of http://news.ycombinator.com/item?id=3460572 , but I feel like I see it all the time)