When it comes to stamps, most alterations are done for monetary reasons, often at the expense of the buyer. A stamp is regummed, reperforated, and so forth. These alterations are not mentioned to the buyer. Ethical dealers do not knowingly offer altered material without mentioning the alteration in the description.
Are there times though when alterations are acceptable? Yes. This happens more with covers, but the same logic can apply to stamps too.
Suppose a cover is in very poor condition. At some point, a sticky substance has adhered to the cover and it is slowly damaging the cover to the point where the cover may disintegrate to nothing. If nothing is done to the cover now, the damage will continue unchecked and eventually ruin an important piece of postal history.
A decision is made to treat the cover to remove the substance so that it can do no more harm. Perhaps the decision is made to also repair missing pieces of the cover that have been removed.
Removing the sticky substance is, in my opinion, not worth noting. It did not alter the cover other than to remove something that was not part of the original cover. Replacing missing pieces of the cover to make it look more attractive is worth noting.
Consider a yellow colored used stamp that is now turning brown due to oxidation. A quick wash in a mild solution of hydrogen peroxide removes the brown color. A purist will say this is an alteration. Is this worth noting to a buyer? No. It did not change the stamp in any way. It improved the color of the stamp and possibly prevented any damage by further oxidation. This is fine by me.
Alterations done to deceive people are bad. However, some alterations may preserve philatelic material. Those alterations are acceptable and sometimes necessary.