Pretty well covered here. If you can get them BONE DRY, then it doesn't matter where you store them; but good luck getting them properly dry unless you have a reversible hydrapak (Can't say I don't want one of those... but I already have an Osprey Hydraulics one that's got a lifetime warranty on it sooooooo). If you're going more than a few weeks between uses, the freezer is a good option and definitely won't bother the plastic.
@Stein isn't wrong, but for me adding some electrolytes a la nuun tabs is a game-changer for my energy level on stiff climbs. Adding sugars to the equation definitely makes proper cleaning more important and necessary more often, but for me it's worth it.