I've met them about three times in different situations, and I've decided that after the buzz of the first time, it's pretty awkward (I'm awkward in general) and it's not necessarily worth it.
They are incredibly nice and with the exception of one time, they have always been generous with time, autographs, pictures, etc. (One time they bolted after the show and kindly told everyone no autographs... the next night, they took over an hour to make up for it.) Lee and Steve are the most approachable; Lee took pictures of me and all the other fans once (told us to pose
) and Steve can talk about music and the band for as long as you stand there.
Thurston often tries to leave early undisturbed, though I have seen him stand outside and talk to groups of people without the band. I guess it depends on his mood. I once gave Thurston a whistle before a show and asked him to blow it during Silver Rocket, which he did after giving me a shout out! It was fantastic!
Kim is the least approachable; though she certainly is not mean spirited, she doesn't seem to know what to do around obsessed fans.
I would recommend going ridiculously early to the show and meeting them before sound check. You'll have to waste the day, but you might make some friends with the other sonic youth freaks, and generally, there aren't as many people jabbing for autographs as there are after the show.