Drywall and similar sheet goods are traditionally installed from the top down, and for good reason. There is
neither rule of law nor physics which would preclude doing it the other way. Are you actually putting board
on the ceiling, or just butting up to it? I do not understand your wife's issue: the tub and base certainly must
be installed before the cement board, and much work while standing in a
new tub/shower base is inevitable. Suitable protective measures should be employed. There are usually good
reasons for putting the board joints one place rather than another, and the layout should be worked out before
work commences. Talk to your wife and get it worked out, nicely.