I am a believer in pre-washing, every thing every time. I don't mind washing smaller pieces but usually cut it off around 10" square. Long skinny pieces like jelly roll strips or old style 8th yards are the worst about putting in the wash and so I usually don't buy the precuts. I wash with like colors from my own laundry and use color catchers to see if I should examine things with more care.
Having said that, I typically don't have many problems with blacks bleeding and so deliberately use it with batiks so it can absorb any cast off from them. Recently I did buy a black solid that was not my standard Kona and I was rather unsettled by the amount of dye that was cast off by the first wash, but it didn't continue to wash out.