Anytime you turn on an amplifier, no matter how long or how short of a time it has been off, it wears on the amp. The power surge when you turn on something electrical would do more damage than leaving it off. Leaving it on in standby promotes wear and tear on the filaments in the tubes and dries out the the capacitors and the frequency response begins to falter.
Want to store your amp properly? Store it where it won't get exposed to moisture, as moisture will deposit residue in the controls. A film eventually coats the inside of the pots and they loose contact with the wiper.
I know. what I am talking about from experience. I have done guitar, keyboard, and electronics repair for music stores for over 50 years. I was an electronic Lab Technion for Zenith Radio when I was younger, and I began repair work at my uncles television shop when I was 8 years old. That is what my comments are based upon, not old wives tales or electronic Internet voodoo magic that some idiots spread around the Internet in order to appear knowledgeable. Take it or leave it. It all boils down to what you choose to believe. Common sense isn't everyones bag of tea.