OK well we now have 2 types when it comes to removing, the older soakable stamps and the newer self stick stamps and there has been topics on how to remove both.
If I get older letters I would leave as a whole cover. If the cover is not worth saving as a whole then I decide if I like the cancel and if so I cut the whole corner.
I get a lot that are out of time frame such as you buy something on eBay
and the sell ships with stamps you want to save but there is nothing historical about the cover. Soaking or removal of newer self sticks you have to decide if its worth the time and effort to remove them. if so do it and if not just store on paper.
What I try to remember is older stuff you cant turn it back into a cover if you remove it and we who have them are in charge of their care for the next generation