I'm a sometimes yes, sometimes no, prewasher. A lot depends on the fabric and what I'm doing with it. If its for a wall hanging that most likely will never get washed, I don't bother. Quilts, maybe. Clothing, always if it's washable. I usually serge the ends of the yardage to keep it from raveling. I hate the mess.
I've never had my machine unbalance with fabric. Most just with heavy rugs.