Why the need for great precision, anyway?
It's always good to be on the more accurate side, but if the aim is to check whether a valve is still worth using, then the bounds needed are quite wide.
For a valve manufacturer experimenting with different constructions and materials, accuracy is important.
For checking that a pair are well enough matched, absolute accuracy is less important than repeatability.
For deciding which valve is best for pace, authority and rhythm, consult an astrologer. They've been in the business far longer
Just for the hell of it, I like the idea of carefully regulated supplies, heater and all, for decent repeatability of measured results free from reliance on mains voltage stability, but I'm honestly not sure what actual difference it would make to the things I do. But then I don't flog valves on ebay.
David