Do tubes have a break in period?

What I have found with both Valve Art and EH KT88s in my ASL Hurricanes is that they take a couple of days to stabilize bias current in a push pull amp. In that two day period, the bias fluctuated as the tubes burned in. This you need to watch closely for a few days before business as usual. I have noticed zero change in sound if kept biased within tolerance. Those who say they "warm up" are probably experiencing ear burn in rather than tube burn in - a phenomenon that makes us think it is getting better, but we are actually training ourselves or becoming acclimated to the new sound.

I had a V12R, Cary recommends 100 hours.

Though the internals of the 300B tube are larger than those of a lot of other tubes; the same components(cathodes/anodes/grids) & dynamics are present in all: (http://www.stereophile.com/content/search-perfect-300b-tube-page-3) (http://www.stereophile.com/content/search-perfect-300b-tube-tale-burn) Read the follow-up of the article as well, for further notes on burn-in(especially paragraph three).

Yes af they are really NOS 50hrs.

I think there's no definite answer for this question. Time is variable.