Completely full would be OK because the liquid won't expand. When partially full, I just squeeze the bladder a bit before putting the cap on to reduce the volume of air in it. Even without doing that, I'm quite confident the bladder would easily handle any pressure differential at the altitudes we fly, and probably a lot higher. I can recall seeing a video of a full bladder being drop tested onto concrete from something like 30m from memory, so they are pretty tough.