As the charge builds on the plates of the capacitor, they actually move a bit in relationship to one another. But, for something sitting around for a few months, that is not the issue.
Still, capacitors take a very long time to come up to song, some types and brands more than others. I know a designer who uses Blackgate brand capacitors, but has put in alternatives for customers who do not want to keep their amps powered up all the time. He told me that his amps sound thin and dry if the Blackgates are not kept charged all the time.
My amp has a lot of Blackgates in them, but I cannot see myself leaving the amp on all the time. For me, it is more a safety concern than life-of-tube issue -- the designer insists that the tubes will last longer being left on all the time rather than being subjected to harsh turn-on conditions.