Well, how much sag makes you worry? Get yourself a 4' length of 2x4 and put some weight on it - get a feel for how
much it might "sag" under the weight of a little drywall. FYI, the big box stores have large piles of "pre-sagged" 2x4's that
you can pick through. Get yourself some saggy ones and install them with the sag "up", so any sag from the drywall will
bring them back down level.