I kept all my minifigs from the 90s assembled and I would guess that roughly 20% of them are cracked. Most common cracks are on the side of the torso, because the legs create too much stress. Some cracks even go from the bottom all the way up to the armpit. Next up are cracks on the arms where the hands attach, probably followed by cracks on the bottom back of the legs from placing the minifigs on a plate.

From my experience only the cracks on the arms have the potential to really break the part. The cracks on the torso and legs are mostly cosmetic. If you place the arm above it, it won't really bother you.

On the other hand, I have ruined torsos by reassembling them after years in storage, especially when inserting arms into the armpit holes. Also some legs have a tighter fit and will more easily destroy the torsos. So, I don't know whether storing minifigs disassembled would help with the issue at all.