[hkstroud]

While you said "rafters", I assume you mean joist. Either ceiling joist if there is a attic above or floor joist is there is a floor above.
My suggestion would be (Now this is going to a bit of extreme but would insure that you have a flat and level surface to hang your ceiling drywall)
First cut bunch of 2x4 nailer blocks. The length of these nailer blocks would be slightly less than the width of the existing joist. Nail or screw one nailer block to each side of each joist about every 48 inches. Cut two lengths of 2x4 long enough to reach from wall to wall along side each existing joist and rest on the wall plates.
Using the longest straight edge you can, determine if the bottom edge of the existing joist are level and even. If so, place your 2x4 lengths on the wall plates and nail or screw to the nailer blocking.
If you find that the bottom edge of one or more existing joist is significantly lower than the others, say 1/4", notch the bottom edge of each end of your 2x4 lengths by that amount. Let's say that you do find that one or more of the existing joist are 1/4" below the highest joist. Notching both ends all of your 2x4's will lower the bottom edge of all of your 2x4's to be even with bottom edge of the lowest existing joist when resting on the wall plates. The addition of the 2x4 blocking and the 2x4 lengths will reduce the spans between the joist by 6" and be well withing the standard 24" span for ceiling joist.

As previously stated this is going to a bit of extreme but should give you a flat and level surface to attach your dry wall. Cost of doing this should be about an hour or so work and 6 or 8 2x4s.

 

[hkstroud]

Wouldn't it just be easier to strap it out and shim as needed?

Maybe, but PMP said he didn't want to use strapping. PMP can take this concept and do the entire ceiling or if the ceiling joist are reasonably level only do the bays that are unusually wide. I know of no requirement to use 5/8" drywall for joist 24" on center. My roof trusses are 24" on center with 1/2" drywall. Been there 30 years and haven't 24'" bowed. Garage is 5/8" for fire protection.
This approach is the simply, no thinking way to insure a flat, level ceiling but does cost a few 2x4s.