Simple question, probably stupid

I listen to tubes I roll as they burn in, and here's what I've noticed in my system: Power tubes and rectifiers 25 to 50 hours, signal tubes 50 or more....all of my signal tubes are mostly 50s and 60s NOS. They all sound good out of the box, but I won't listen critically until about 50 hours

Hello mazian.  It is important not to operate a tube amp using output transformers without a load on the output terminals. (As opposed to solid state amps which don't care about having speakers connected.) The advice of others responding before me is quite useful. So if you decide to play music while putting hours on the tubes (not really necessary, as they won't sound "bad") substitute a suitable resistor for the speakers if you don't wish to hear whatever is playing.  I encourage to play your favorite tunes and enjoy the music.

Tubes don't burn in but your hearing gets accustomed to the music and tricks your brain into believing there have been changes. Interestingly, literally every example that I've read of something "burning in" has resulted in better sound. How could that be?

Mho, there are no stupid questions, really.
Inspired to inane answers, yes. 

I've viewed 'break-in' as something that should be done with ss as well; 'conditioning' to a life of service...

I recall when for vehicles of any sort called for a break-in period of +/- miles before one did 'pedal to the metal', to let all the mechanicals 'seat 'n seal' properly.

How about a 'structured approach'?

10 hr. 'idle'; on with no signal....just warming up.
10 of white or pink noise, low volume.
10 of low volume program, 10 of louder, etc.

That allows for some degree of 'grats', even if not 'immediate'.... ;)

Can't hurt, might do the thing that's suggested without having to 'pause the Cause' for 100... ;)

Enjoy the 'arrival', J

I have always found that tubes do get better with "burn in time". As others have said and as Brent Jesse who is my go to for NOS tubes says you have 2 options: leave the amps on for the desired period of time, no signal needed or just burn them in by using them. I've generally done the latter as I really enjoy hearing how they change.