As other have said, it depends. High value stamps shouldn't be hinged under any circumstances (particularly with modern hinges), even if there's already a hinge mark. Stamps valued at Scott minimum probably aren't going to take much of a hit in real world value if they're hinged. I've hinged many low value NH stamps over the years, but recently I've shied away from that. I'm not worried about resale value, but I kind of look at myself as a caretaker of the stamp for the next generation of collectors.
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The catalog values for modern stamps are largely just formula based: Scott uses "twice face value" as the default, for instance, and same CV for mint and used, and those prices sometimes aren't updated for decades. They don't reflect actual market transactions.
Yep, this is particularly true for many modern used stamps. Depending on the country, they'll value used stamps at 2x face same as mint, or sometimes they'll value mint at 2x face and used at 1x face. In many cases, the used versions are realistically penny stamps as they're plenty common. As another example of how Scott prices for recent issues don't reflect market pricing, you can look at Japan, where Scott automatically assigns most used issues a value of about 1.5x face. 82Y issues (the current letter rate) are valued higher than recent 52Y & 62Y issues (the current postcard rate). At least in the kiloware I've purchased, used postcard rate stamps are a couple orders of magnitude scarcer than letter rate stamps and usually sell for a higher price, but Scott assigns the higher value to the letter rate stamps.