You're worried about getting 1" of standing water? In your upstairs? You've got bigger problems than damp sheetrock if that happens.
Planning on cutting a 1/2" strip or more off of every sheet?
Normally the ceiling is done first and carried all the way to the walls. Then the upper wall sheet goes on horizontally tight against the ceiling. This helps support the weight of the sheets on the ceiling at the edges, lets the ceiling sheets have adequate framing area to attach to at the edges, which helps prevent future nail/screw pops, and allows fewer fasteners to be required. Of course using adhesive helps as well. Then the bottom sheet goes on up tight to the upper sheet. Whatever gap is left over at the bottom is what you get.
Leaving a 1" gap at the bottom also means you've got less room for fasteners. You'll have to be dead on for spotting them and very close to that edge you cut off, meaning breakouts. If you started at the bottom with a full sheet then you'll have to cut the top sheets at the ceiling, which means gaps, inferior support of the ceiling edges, and more work to do the mudding.