Anybody own a hydration bladder?

I'm thinking of buying Platypus water bladder, but I wasn't sure what capacity to get. So my question is if any of you have one, is it possible to just fill it halfway? Obviously I'm sure this is physically possible, but I mean like if a bag's compartment is too small to fit the bag when completely full, will filling it partway and then "crumpling" it a little bit to fit still work, or do you think it would mess up the flow at all?