Is it a OK to leave tube pre-amps powede up 24/7

I follow the on once, off once, per day.

There are only so many electrons in a vacuum tube. The longer you burn the tubes, the shorter their lives are going to be. You can expect to wring about 10k hours from pre-amp and driver tubes, and about 3k hours from power tubes. An important issue is how you bias your power tubes. That will have a major effect on their longevity. Yes, a lower bias in a class A/B will create a lower transition to Class B, but you probably won't hear it at lower volumes. This is not an issue with signal tubes used as drivers and in pre-amps: they are running full-tilt all the time regardless of volume and tone settings. The volts applied to them will affect their lifespan, but that's not something you can control from the front panel. 

Well Atma is spot on as usual.  It depends on design. The tube preamp I have owned for years, TRL, ran barely warm and was a better climate for the parts inside.  The designer always told me to keep it on unless traveling for days etc...

This actually led to better sound, happy internal parts, and longer tube life according to the designer.  

I have never heard of a tube preamp catching fire and Atma's comments cannot be improved on.  

I run a couple of hybrid amps and both have circuitry that allows the tubes  to be turned off when you switch to stand-by mode.  It means a delay when you fire it up again, but not as much as if you turned it off completely.  One is an Audio Research and the other is a CJ Premier.

Keeping your tubes fired up all the time will drastically shorten useful life.

I have a Rouge integrated amp with stand-by soft start. Truthfully, after a half hour warm up it sounds great and additional hours of warm up seems to make no difference, so, except for periods when I plan to listen to music for a couple of days in a row (even then usually turn on in the am and off at night unless on an all night music binge) I see no point in leaving it on 24/7. I also have a Audible Illusions Modulas II preamp, also with a stand-by soft start and left it on continually only during periods of extended listening and off during periods of not listening. I had to change out the tubes only twice in 25 years. If your pre amp has a soft start and you listen to every day, I agree with paul79, on once off once each day.