Using 8' or 12' sheets, an average wall will result in what? One butt
joint on an average wall vs. one EVERY four feet if laid on
vertically?

The walls of most rooms are between 12 and 16 feet, which only translates
to two or three joints when the sheets are hung vertically.

In any case, we hung sheets vertically on the 28' wall of our garage, and I
can't see any waviness anywhere. That's the advantage of having the tapered
seams where the sheets meet. You can get a perfectly flat joint if you take
your time when taping.

put in the ceiling. Do it now!

I agree, I'd do the ceiling first, even if that's all you can afford to do
right now. It's much easier to move junk off to the side to do a wall than
it would be to move everything around to get to the ceiling.

Anthony