While technically a good rule to follow, I can say I've reused them plenty in certain applications and never had an issue. I do always inspect them carefully.
In noncritical applications I don't have any concerns reusing them if they are in good shape.
As brake pad retaining pins I don't have a problem reusing them a few times since they aren't spread very far and metal fatigue isn't as much of an issue.
For tie rod ends, axles and applications like that I don't reuse them.