My reasoning is I never add anything to my tap water that I don't have to. I generally use salt when dealing with issues that could cause osmoregulatory issues, such as stress from transport, relocation, etc, as well as what you mentioned, injuries. Adding salt to freshwater reduces osmotic pressure. A fairly straightforward read on this subject:
https://www.angelsplus.com/ArticleOsmosis.htm
But using salt (or anything else for that matter) as a preventative doesn't make any sense to me. A fish shouldn't generally be under osmotic stress, and if it is one should be taking a closer look at their set up, and removing the stressors, not salting them down. I hope that makes sense.