I_mine_cor and APS brought up a very good point.
I've always tried to maintain a happy medium when critting - if a song is bad, I'll tell someone. I've given out plenty of 1s and 2s over my time here, but at the same time, I'll also point out where they suck and what I feel they can do to improve it. If they respond well, then I'll +rep them and make an effort to crit their songs in future. If they get narked and start getting funny, then they get neg repped and I won't touch them with a bargepole.
What upsets me is when you see the guys (and they are about) that come on here, follow the rules, give three or four good crits to people, and then post their song only to watch it slide down the first page with no crits... THOSE are the songs we really need to aim for, because chances are, they are the decent guys that will stick around.