Like the other said, just be sure the BBs are clean and you don't scoop up anything else and put it in the bag. Washing isn't necessary so long as they are clean. If you have to wash your BBs before you, consider a different brand. BBs should be clean out of the bag. Some people used to clean the graphite BBs because they were powdery, but I think those went the way of the dodo.
Referring to some other posters, reusing BBs is generally a bad idea as they do malform on impact by design, so they are no longer spherical. Used BBs will potentially jam your gun or rip your hopup. Used BBs in a grenade might not be bad but you still risk jams.