Yes, that is why most valve rectifier amps don't have standby switches 😀
A valve rectifier warms up slowly with the rest of the tubes, not the case with a solid state rectifier as it HITS the valves hard before they get time to warm up, hence the standby switch. This allows the heaters to warm the tubes up before applying the B+, which reduced the jolt the valves get, which does indeed reduce lifespan and can lead to what is known as 'cathode stripping'.
RE leaving the amp in standby when not in use still reducing tube life, yes, because there is still voltage on them in the way of heater voltage on the tubes it will cause some wear, but not near as much as hitting the amp from a cold start with the amp in 'play' rather than in 'standby' mode.
This is the main reason I put a 'Standby' switch into the Champion 600 as part of the mods.
Hope that helps!
JP Le Roux (an amp tech) told me it is best to switch off whenever not in use and that the standby thing is just to prevent the tubes getting hit hard with voltage from the solid state rectifier when switching on initially, and that leaving it on standby still reduces tube life.