I wash everything as soon as it comes in the house - EXCEPT kits and pre-cuts and sometimes really large pieces (they get washed as they are cut into manageable sizes). I've never bought a BOM, but would definitely not wash it.
Mitty said it very well. There are lots of reasons for pre-washing, but it's a personal choice. One of the downsides that I've run into recently is that many fabric exchanges require that the fabric not be washed. That means I don't participate, because I don't have unwashed fabric in my stash, and I don't want to receive squares or strips of it from others.