Leaving the amp on standby isn't the problem, it's turning the standby off (or turning the amp on, I guess). When you click the standby off, there's a surge of current thru the tubes which can wear them out a little faster. Not a big deal in general, but you don't need to use the standby, just turn the amp on/off and leave the standby in the on position.
The REAL reason the standby switch is provided is to switch OFF the high voltage (B+) UNTIL the tubes reach their FULL OPERATING TEMPERATURE, so as to avoid “cathode stripping” of the indirectly heated cathodes of the preamp and output tubes. When you power up the cold amplifier, it should be left in standby mode for 30 seconds.
The standby switch should NOT be used as a 15 minute, go get a beer, switch. If you are taking a break, turn the amp off. You don't need to put it in standby mode before you turn it off.
This info is per Fender, Marshall, Mesa Boogie, etc.