If you are buying good quality fabrics (except batiks), there is generally no need to wash any of them if your only purpose is to prevent bleeding or shrinkage. Fabric production has greatly improved over the years & it's almost never necessary for those purposes. Pre-cuts can often come a tiny bit unraveled in the washing process & since they are already so small, fabric companies typically advise against washing pre-cuts.
Because I have severe allergies & chemical sensitivities and also only make baby quilts (and babies also have sensitive skin), I always wash ALL of my fabrics when they first come into my home. I toss them in on the hand-wash cycle in my washer on the coldest setting (Tap Water Cold) with a couple Tbsp of Tide Free. I have occasionally actually washed them by hand, but only if I really need every last bit of the pre-cut because that's a lot of work for very little difference. I generally toss them in the dryer after clipping any stray thread, on the Low heat cycle. If it is a very tiny amount, like a charm or mini charm, I will lay them out on a drying rack instead.
In addition to allergies, if you are planning to work with fusible web, many of them work best with the sizing removed (by washing). In that case, regardless of the size of fabric, I would always suggest pre-washing.
Other than those two reasons, there really is no reason to ever pre-wash anything except batiks & lower-quality fabrics (and maybe some super saturated red quilter's cotton ... although I've never had any issues with Moda or RK Kona reds bleeding).