Sagging rafters can lead to serious problems with the entire roofing system.
How to level roof rafters.
In order to build rafters you need to know the roof pitch and the roof span overall width of roof.
If needed shim trusses or rafters to level the nailing surface and install blocking to straighten any warped or bowed top chords of trusses or rafters.
Mineral wool friction fits between rafters providing an optimum seal and preventing gaps associated with rigid foam boards which can otherwise lead to unwanted heat loss.
If one support is as little as 1 4 inch lower or higher than the one next to it you might end up with an uneven roof.
If your rafters are still sagging a bit and it is causing your roof to dish out a little in the center of the roof sides add a 2x4 that goes all the way across the rafters in the center of the length of the rafters.
It is well worth the effort to rough out the wall system to be square and level so the roof framing is simple.
Use your string line to determine how much the roof need to go up.