Since we're only talking about non-power tubes, then neither of your questions should be a source of worry. Leave the tubes powered 24/7 and change them at fixed intervals. It could be every 6 months or 2-3 years. A very small number of vendors are selling truly unused NOS tubes, most are used, even when stated otherwise. If you're not hearing any distortion or gain imbalances, then don't worry about it.
How can do you extend the life of your tubes?
My amp premp and cdp are tubes. I read years ago, proportionatley, the most tube wears comes from powering up. And shutting down, and then powering up shortly is life shortener for sure. Shortly is too vauge. What do you tubers do to lenghten the life of your tubes, especially when we're talkng NOS and the like? thanks in advance.