Over tightening enlarges holes more than properly torquing. Extreme cases will crack crank around hole and repeated mounting/unmounting will result in crank bottoming out on BB, this can happen faster if overtorquing.

Undertightening then riding 'em loose causes hole to be not-quite-square. This renders them Un-tightenable. They might seem tight for a couple miles but will eventually creak and get loose and hole will become even more un-square.