In a recent issue of the American Philatelist (the monthly publication from the American Philatelic Society), there was an article from the curator of the Royal Stamp Collection. In it he stated, and I see evidence of this in many many stamps, that there is going to be a time, and he believes it has to be soon, when ther matter of gum needs to be decided. The gum on most stamps is acidic, and for stamp preservation purposes, it is going to be necessary to soak the gum off of all stamps, or there won't be any stamps left.
The APS, other philatelic organizations and the catalog companies need to take a serious look at this. At the high price of buying never hinged, we're actually destroying the future of all stamps. Read the intro to Iran in the Scott catalogs. Gum condition does not detract from the catalog value. The first self-stick stamp from the US, the Chirstmas weather vane stamp is deteriorating, and the catalog value is for discolored stamps. (Don't soak that stamp off of paper, by the way. It was printed in a manner that the paper separates and falls apart when soaked.)
In every collection that I see, unused stamps from the 60s and earlier are turning brown, It doesn't matter iof the stamps were stored properly or not. The gum is reacting with the paper. Also, much of the paper that the stamps are printed on is acidic, and I have seen many stamps just crumble into pieces.
The hobby has a crisis that needs to be addressed.