Replace tubes yearly?

1. Always keep spares for when eventually will be needed.
2. If you hear noise or distortion other than normal a bad tube or tubes is suspect
3. To find the bad tube replace one at a time and listen for a difference
4. A tube tester is your friend but how many people actually have access to one these days?

I bought a tester when I started using amps with rectifiers and four pin antique tubes.   It save me from disaster with my 300b.  A rectifier took out the fuse on my tester (basically a small light bulb) and not my amp.   

Not practical to own/buy  one unless most of your gear uses tubes.  I had a guitar amp repair tech near me where I used to live and he sold tubes.  He was happy to test tubes , often times he would end up selling one or two. 

I wonder how many tubes get thrown out because they test weak on a tube tester, when they could perform perfectly fine in a cathode follower circuit.

Which tester to buy and how to interpret the results is a very complicated subject unless you are just looking for rough results, like avoiding shorted tubes or dead tubes.  I had tubes that tested very weak on an Amplitrex (modern, industry standard tester) but still sounding good.  I contacted the amp manufacturer who said that in this particular application weak tubes would work fine and I should use the tubes until they sounded bad or died.

@invalid good point that not all tubes need be top performers. It all depends on the specific design of each amp.

I had a ARC sp16 tube preamp that used 6 12ax7 tubes. Only one or two required very high performance tubes. When I bought tubes from ARC, 1 tube always cost more and was designated for use in a specific slot. The others cost less. If I heard noise I would swap out the bad tube but keep it as a spare in that it could be perfectly fine next time. I labeled my used tubes in a hierarchy so I would know which ones sounded best still when removed and would always try that one first. My stash of spares that were removed prior kept me going nicely for many years. That one slot that required the top notch quality tube was replaced just once with a replacement purchased just for that from ARC. Just an example. I’m sure every tube amp is different. There may be cases that require all tubes be replaced once a year. Some amps are harder on tubes than others. It all depends. Tubes can be quite cumbersome and expensive to maintain, especially if there are lots of them, which is something I have always tried to avoid, but of course different strokes….