If you are worried about absolute calibration accuracy you will need to check it every 6 months and go through the entire procedure every time the rectifier is changed (or degrades significantly) and I doubt if you want to pay the ebay price plus shipping for calibration that often to maintain high spec calibration.
Especially true when you consider that a tube tester guarantees very little. It's great for finding shorts or gassy tubes, and it
can get you in the ballpark for emission, but that's no guarantee the tube is good. I had a brand new JAN spec tube used as a replacement in my 75A-4 a few years back that drove me nuts. My B&K 747 showed it to be fine - no shorts, no gas, excellent emission. It just wouldn't work properly in the circuit. After wracking my brain for hours (because I
knew it couldn't be any of the new tubes I'd installed), I finally smartened up and started swapping out the tubes I'd replaced. Voila - problem solved.
If you buy a really scruffy tester, it's worthwhile to go through it to replace aged components or have it gone through and calibrated once. After that, just perform the typical maintenance and touch up if you change the rectifier or anything else and call it good 'nuff.
