Why? Unless your gym has a ton of squat racks, you're probably impeding someone elses workouts unnecessarily.
Straight bar curls are also much worse on your wrists.
I figured by now a recommendation to curl in the squat rack would be viewed as facetious.
The part about the straight bar though, to my knowledge, is true. I believe this is also covered in Starting Strength in the 'So you're gonna do curls anyway' chapter.