How far down should sheetrock come in a shower tub installation?
Ideally, drywall should be in the next room, or at least around the corner. Tile backer board (not green drywall - cement-based products in 3x5 sheets) is much better for the job. If you think you simply can't do that, stop the drywall at the top of the tub surround or shower enclosure and switch to backer board from there down. And use moisture-resistant drywall, at least.