I don't prewash. I'm too lazy! There, I said it!
But seriously, I asked the lady in my LQS and she said that it's not necessary any longer with new fabrics. However, if it is a very colour intense fabric (e.g. a red batik) she dunks the edge into a glass of boiled water, if it doesn't shed colour, you're ok to use it like this, otherwise pre-wash. She advised me to just iron the fabrics with lots of steam as it's apparently the heat that makes fabric shrink. I do that now.
And if I feel that the fabric is a little flimsy and not stiff enough for my taste I also starch them (before cutting and sometimes when pressing seams as well). Also, when I know that I'll be working with bias cut pieces I use more starch.