Spuff,
As both a collector and a dealer, I hate gum. Well, I hate gum on the old stamps. If there is gum on a plus 100 year old stamp, most likely it has already discolored, crackled and may have even lead to foxing of the edges (perfs). Fortunately in Mexican stamps few of the collectors care one way or the other if there is any original gum on the stamp. Not so with other countries.
I once purchased a beautiful US #26 three cent issue in mint condition. The gum had album paper and hinges stuck to it and generally looked bad to worst. I was faced with either removing the gum or likely have the stamp returned with its awful looking back. So, I gave the little guy a bath. Came out beautiful, but now catalogued a small percentage of original gum catalogue value. I have often thougth about that stamp as one that needed to have it's gum removed but should have been worth every penny of what a full gum example should have sold for. I find it really dishearting that it may have gone to some guy that will regum it and sell it as MNH for a huge price to some unknowledgable collector.
Now I'll get back to soaking some more gum off my Mexican material.