I've owned a Hickok 600 for about 12 years now. It was freshly calibrated when I bought it and I haven't had it updated, but it doesn't appear to have drifted too much. I test every tube I get and record the readings, so that I can both compare them after a period of use and compare them to other tubes of the same type. That system has worked well, and I believe mitigates the effects of any change in calibration. I rarely resell tubes so absolute accuracy just isn't a priority for me.
That said, markus is correct; if you're buying one it should be calibrated and completely functional, and that doesn't come cheap these days.
That said, markus is correct; if you're buying one it should be calibrated and completely functional, and that doesn't come cheap these days.