I pre-wash everything. It saves lots of heartache later. I also buy a lot of fabric remnants from various places (bazaars, markets, charity shops....). I like everything to be clean and fresh prior to storing with my stash. Also as GMA says, it does remove sizing and any other chemical residue from new fabric.
I bought some fabric remnants yesterday...a few baby print flannels and about 3 metres of dark blue fabric with bears and stars...whilst the fabric was unused, it did smell pretty awful (A huge bagful for $2). A quick wash and out on the line, and voila...clean, fresh, and didn't run so now it's ready for my next project. I also visited a fabric store recently and purchased some fabric off the bolt. Again I washed before storing. I am often amazed at how dirty even new fabric can be.