If I have ceiling joists that are 48" apart, will 5/8" drywall sag if
mounted on it?

I'm not seeing too much information about that around, even though I'm
pretty sure 48" is pretty standard framing for a basement.

I made the mistake of putting 1/2" up the first time - bad bad mistake that
cost me dearly. But 5/8" isn't that much more. The idea of installing
2x4's between the studs isn't my idea of a fun time, especially since it
will introduce more margins for error unless I'm a demon for measurements.

I am also considering the idea of using plywood backing over the joists as a
way to give the drywall a larger surface to attach to - even if it isn't
structural it will still remove any sagging. But that option has the
detraction of reducing my headroom by 1/4" or more. I know it doesn't sound
like a lot, but it all adds up - not to mention it will make adding lighting
fixtures a huge pain in the ass since you can't cut plywood with a drywall
saw.