I prefer matching socks, although to be honest, I'd rather not have socks at all. I hate them. I only wear them when my feet are cold...otherwise it's sandles all the way.
I also can't say I'd care overly if I only have mismatched socks either (unless I was going somewhere which I didn't want people to see). Normally I'm just at home and don't care anyway.

Can't say I'd ever buy mismatched socks, although it seems like a cute idea for a kid