I used to empty and dry my camelback bladder. I even made a fan arrangement to dry the residual water. Now I just leave it with water in it after use. When I need to use it again, I'll dump the old water and fill with fresh. I figure it reduces whatever might be leaching into the water from the plastic. When It fill it, I suck the air out. I know some mold will grow in the absence of air but so far, I have had no visible growth in the bag and it has been at least three years since I've washed or dried the bag. YRMV.
'03 KTM 450EXC
"If you know what you're doing, then you aren't learning anything." - A. Lincoln