As auctioneers, our experience has given us the following answers to this question: The effect of hinging on mint modern stamps is more significant the further you go back to the 1940 cutoff that the Scott catalog uses. Stamps from most European and British area countries in the 1940-60 period sell, in hinged condition, for 1/3 to 2/3’s of their NH price. As one moves closer to the current period, the ratio of hinged to NH price gets closer so that when we sell mint collections of the last twenty years, it doesn’t seem to matter much if the stamps are hinged or Never Hinged.
I suspect this is for two reasons. First, the quantities of mint stamps from the last two decades is small because so few were put away, and they haven’t recycled through the philatelic resale market yet. Because so few were put away, collector demand is high, and so hinging matters less. And second, the face value of modern stamps is a big determinant of the price, with the Scott policy that many modern stamps catalog at twice their original face value. With stamps that have a high postage value relative to cost, hinging doesn’t matter at all as you can apply a hinged stamp to an envelope as readily as a never hinged one.