These are all expensive collecting methods and I don't think they add much to the enjoyment of the hobby. You get the benefit of using the stamp at face value and you get to collect it when it's done with it's job. The important thing to remember with used stamps is to cut them off the envelope leaving a bit of paper around the stamp. Do NOT peel the stamp from the envelope as this will damage the stamp. Used stamps should be soaked off as described below.
Water is the best method for removing the stamps from envelopes.
To soak off stamps, get a shallow tray which can hold about an inch of water. Put about an inch of cool or even luke-warm water in the tray and place it on a flat surface. Leave them soaking for 15 to 20 minutes. After this time, you should find that the stamps slide off the envelope paper quite easily. Some will even pop-off themselves.