I'm not a prewasher, never had any problems by not prewashing. however if you do decide to prewash, use the settings you expect to use to wash the finished item. I know many people wash on HOT for shinkage etc. but if you read the ends of the bolts for care inst. you will see warm/cold is recommended for many fabrics . IMO if you should follow the manufacturers recommendation for handling their fabric for the best results.
But again, everyone has their own ideas based on habit or experience.