The issue of cathode stripping is only applicable to transmitting valves operating in the kilovolt range, it is not relevant in receiving valves operating at a few hundred volts.
Standby switches on guitar amps came about when Fender included one on the Fender Bassman amp, there are a number of theories as to why this was done but they certainly do nothing to extend valve life and there are good arguments that they actually do the opposite. Of course when Marshall copied the Fender Bassman for their JTM45 amp this meant two of the major guitar amp manufacturers were using standby switches and they became common on many guitar amps despite the fact that they are completely unnecessary.
Some good information on the subject here
http://www.valvewizard.co.uk/standby.html