I posted about this in detail when I did the downstairs... but a quick recap. Old construction lumber can vary a great deal in dimensions. When they built the floors, the joists were leveled to the floor surface leaving the ceiling side to vary across the room. It then became the plaster guys to "level" the ceiling. In the world of drywall this is not an option, so there's no choice but to sister the joists. I read a few years about using a laser to find the lowest joist in the room and then running string across the room at the level. Then using metal studs which are virtually guaranteed straight, sister them to the joists at the string lines. This system worked great downstairs so I'm using it again.