300Bs - does leaving them on shorten their life?

@Vineland,
Wise decision. Some people can make an argument for leaving the small low signal tubes on continuously (Although I don’t). I can’t imagine any reasonable case for keeping the power output tubes on 24/7, none. Now if your amplifier has a very low draw ’stand by’ mode that’s a possible exception.

Good quality 300b tubes have long lives (The EML XLS and vintage Western Electric 300bs have reported 40,000 hour lifespan) but I wouldn’t gratuitously waste useful tube lifespan leaving them on constantly. If I’m home all day I leave my 300b amplifiers (Mono blocks) on through out the day if  listening sessions are planned  and then shut them down at the end of the day . I do this rather than turning on and off multiple times during the day.
Charles

@vineland, 
Via another thread I discovered that you have the VAC Renaissance 70/70 amplifier. That is a superb sounding amplifier, congratulations! One of the best sounding VAC amplifiers I've ever heard and a genuine classic. 
Charles 

lol

Some equipment has built in protection.

My Cayin  tube Amp has a 30 second delay, test and I suppose you could call it a soft start.

My prior McIntosh SS amp has a test/delay protection sequence.

My Adcom ACE-515 power enhancer (not a power conditioner) power controller (in my office) has a time sequenced 10 sec turn on delay for the amp; 30 sec turn off delay for accessories. (prevents thump to speakers).

tempted to turn on and leave on all day. Instead of powering up and down many times. 

Correct answer depends entirely on what you mean by "many times". If you mean many times a day, the greatest most intense stress is the thermal expansion going from off to on. This is why some designers like Herron incorporate slow warmup circuits. Greatly extends tube life. 

But if "many times" means one time a day? Well then look at it this way. Let's pick some arbitrary numbers and say tube life just sitting idle is 5000 hours. At idle. Playing music uses more power, tubes get hotter, wear faster. Let's say you crank that thing full blast all the time. Tubes last 3000 hours. Just throwing numbers out to get you thinking in terms life and wear. Though these are probably in the ballpark. 

Okay so now every time you turn them on from cold, that flip of the switch is something like at least an hour of running idle. So if you are turning it on and off 3 or 4 times a day then probably better off just leave it on. And that's how that works.