I always wash my fabrics before I use them. Not only for sizing and making sure they won't bleed/fade or whatever, but also to wash away the chemicals used in manufacturing. Also, I think about all the "hands" that touched these fabrics before I bought them, washing them gets rid of any germs or dirt that may have got on it (can you tell I work in health care lol?). Washing them gives me a peace of mind and an easy feeling while working with them.
Just my two cents.