I prewash, almost always.
I don't prewash any pre-cuts, that's a recipe for disaster.
I don't prewash fabric that I'm going to be cutting into for a stack & whack, it's too hard to line everything up after washing.
Sometimes when I'm in a huge hurry, I won't pre-wash yardage - but I always feel it's very risky and don't like doing it that way.
I also don't like the way unwashed fabric feels.
But by and large, I'm a pre-washer. I want to make sure the fabric behaves, but more than that is the fact that I want to remove as many chemicals from my environment as I can. I handle fabric a lot as I'm sewing it, so I want to get rid of all the pesticides and sizing and who knows what else it's been treated with on it's long journey from the cotton field to my sewing room!
I try to pull the fabric from the dryer when it's still just a little bit damp and drape it on a drying rack to let it completely dry - that makes a big difference with wrinkles. Then I wrap it on comic book boards into little mini-bolts for storage. It is a little bit of work, and I often end up with a small mountain of fabric waiting to be "processed" after splurging at an expo or while on vacation, but that's a GOOD problem to have!
Honestly the hardest part of the whole process is getting it nicely folded on-grain lengthwise, but I've gotten really good at wrestling big hunks of fabric by myself. Practice makes perfect!
