After your done with the mix and you bring the stereo track into a new project for mastering should you set the timing the same as in the mix project or keep the default 120 in the master?

I've wondered this myself.  Invariably, I leave it at the default 120 bpm and the project plays fine, but is there an advantage to changing the tempo to the actual project tempo?

If you decide to use tempo based FX or want to snap automation to Measure/Beat it can be helpful to have the time correct. I would set it... even if it's a live recording made without a metronome and your want a good approximation.

It doesn't matter in any way except possibly for time based delays and such (which you're unlikely to use during mastering). You could set it to 9 or 999 and it shouldn't make any difference to the audio playback.

I didn't think it mattered. I never noticed any difference in time.

Setting it to the correct tempo makes it easier to position markers.  It's not helpful to have wrong bar intervals displayed.
 

So which is it? Should bpm in the master be set to what the project is or not? This is conflicting.

There are potential advantages to keeping the same tempo (e.g., if you need to add markers or snap automation nodes to tempo), so that's what I'd recommend. However if the tempo isn't the same, it's very unlikely that will be a deal-breaker in terms of completing the mastering process.

In other words, if it's easy to type in, go right ahead, but if you need to fuss a lot to find the tempo, maybe don't bother?
EDIT: It won't make any difference to the audio playback in any case, it just may improve your workflow in some cases.

...or, to put it another way:
If you intend to make use of the tempo, then set it.
If not, then don't bother.