I'm not sure what the norm is, but I have purchased 5' screws that were pretty darn straight. Bought from Roton.com....
On the other hand, if you can get it close to straight it should not be a problem.
It's gonna be constrained by the AB nut, so it should not be able to wobble too far. On my first machine, I have a 1/2" screw about 48" long and while it is straight, it still whips a bit at higher speeds. I don't think a 24" screw will be any problem at all.
HTH
Jeff
"I'm tryin to think but nothin happens" - Curly Howard