I save the left over thermal compound. If the applicator is sealed the compound should be okay. I have a nice collection of different brands that I have tried. I use the left over compound when building pc's for friends, neighbors, and relatives.
There have been some reports of AS5 possibly deteriorating after prolonged use. The recommendation was to remove the cpu, clean it, and reapply the compound. I no longer use it so I do not personally know if that is true.
werxen - The manufacturer of the compound I have been using for the past two years claims it will not deteriorate: http://innovationcooling.com/thermal.htm