agreed, it's nice to have a small bottle for around camp use when not wearing the pack with the bladder in it.
my wife only lets me store only so much gear in the freezer (a great place to keep hydration gear from growing things as a prev. poster stated), so on one occasion:
successfully recovered a mildewed (is that the right term?) tube by filling with bleach/water solution (sorry didn't measure bleach - but there was a lot). let it sit for several hours and then rinsed...and...rinsed...and... - you get the picture.
in the end no mildew/fungus and no bleach taste. [while i spend a lot of money on gear, i rarely throw anything away (pack rat syndrome), preferring to fix, patch, or repair if at all possible.]
have used compressed air to blow the water out of the tube.
have a neoprene tube cover for below freezing temps. it, combined with drinking every 10minutes, keeps it from freezing. also, if one is so inclined, i would guess that electrolyte sol'ns might have a lower freezing pt than water??? can anyone out there tell us if this is true, or am i guessing totally wrong here? really, have no clue as to the freezing point of "Gatorade" or equivalent electrolyte sol'ns.
i, however, no longer like to use electrolyte sol'n in my bladder; now only throw-away Poland Spring Water bottles - more possibility for fungus growth in the tube. too much trouble to clean as i once found out.
of course, all water sleeps with me inside the bag in cold weather. sometimes it begins the night as a warm or hot water bottle/bladder. yes, it uses some fuel to do this, but it's so nice to "tuck in" at night. just make sure all water containers don't leak. not all the time, but if i find a leaking bottle cap, have used a sandwich bag over the top with a rubber band securing the bag to the neck of the bottle. never had a BigZip open up, but it's possible.
after cleaning a BigZip, and drying with an absorbent cloth (even a paper towel - these work real well), loop the tube and insert into the mouth of the BigZip to hold it open for air drying. under most conditions it will dry faster than fungus can grow.
store the tube with the bite valve off.