I throw out all my socks and get new ones every so often. When I get new ones, they're all the same sock. I couldn't mismatch my socks even if I wanted to since they're all the same. 😛
I only do it by color. I only have black and brown socks, and they'd look pretty stupid together. I honestly couldn't care less if I was wearing one Under Armor and one Nike sock.
I try to put them together but if I can't find them I just grab the first two I can find. I try to make them at least the same length, but I can't say I haven't gone out with complete mismatches before. Good thing I tend to wear jeans and closed shoes so I can get away with it.
Sometimes I do, sometimes I don't. For work I always do match them as I wear trouser/dress socks and they'll show. And for exercising as I have a few pairs of specific athletic socks I wear for activities like running and hiking, they have extra arch support and overall comfort.