I haven't been leaving air in the ziploc bags. Think of a really full bag and imagine poking at it with your hand (not with something sharp). The skin is under tension and the water can't move around in the bag, so the pressure increases and the bag may burst. Now do the same thing with a 3/4 full bag (no air space). The bag can easily change shape so I think it's less prone to breakage. The 1 liter bags are pretty hard to break. This is more of an issue for the 1 gallon ones.

I notice these bags aren't as easy to find as before, and they don't come in as many sizes as they used to (haven't seen 1/2 gallon or 2 gallon ones lately). Maybe I'll stash away a couple boxes.