Alterations lie on a spectrum of acceptability to collectors. It would never occur to me that I might have some obligation to inform a future buyer that I wiped a fingerprint or a smudge off of a card prior to submitting it for grading. Yet, there are those in the hobby who literally wish to make such demands of others (Sports Card Radio is one -
https://www.youtube.com/watch?v=MI4B8lfebdE starting around the 3:40 mark).
Likewise, it would never occur to me that if I had a card with a dinged/lifted/bent corner that I pushed back down prior to selling that this is something that ought to be disclosed. Honestly, I find even the mere suggestion to be pretty damn funny.
Everyone has a different dividing line for what they deem as acceptable and unacceptable. Nobody cares to place demands to disclose on condition improvements they deem as acceptable, only for things they deem as unacceptable. But where that line gets drawn is different for everyone. So who gets to decide for the masses?
At the end of the day, it's a dog eat dog world out there, especially in this hobby. The reality is that ~nobody is going to disclose this stuff, so you just need to educate yourself and look out for your own interests, whatever those are. If you have a card in hand and you are unable to detect any alterations whatsoever, despite having years of grading experience at one of the top grading companies, then what difference does it really make if something was indeed done to that card? At some point, we just become the old man screaming at clouds. Don't scream at clouds.