I personally pre-wash everything. I don't do it because I worry the fabric will shrink (that hasn't been my experience with modern LQS fabrics); I do it because I have a severe allergy to the chemicals that manufacturers put on fabrics before shipping them to stores.
Little pieces (jelly rolls, charm packs, etc) I usually wash by hand & lay flat to dry. Other pieces I put in the washer on the Intermittent Hand Wash cycle & I tumble dry on low heat to partially dry. Sometimes I do put small pre-cuts in the washer & dryer, too ... it depends on my mood & how quickly it needs to be done.
I know other pre-washers who do so in order to pull dye out or to cause to shrink as much as possible. In those cases, they have to live with the pre-cuts unwashed because putting them through harsh conditions would completely defeat the purpose of buying a pre-cut as they would no longer be the correct size. However, I do it all the time under gentle laundering conditions & measure the pre-cuts after laundering; so far I only had 1 piece that shrunk (or maybe was a miss-cut) and had to be thrown out.