I used steel reels successfully back in college, but that was ages ago (and it's not a daft question). Since then, plastic.
Without going into too much depth, while I can continue to use plastic reels where I have my darkroom, I want steel reels for another location with less storage space (I can load film in steel reels while still wet from a prior roll).
But I'm really just curious as to what is an acceptable tolerance. Though, as Jason said, it might be more how they are bent vs. how far.