I'm currently upgrading my bikes group set from 105 to ultegra... my fiancé's bike currently has a Sora group set on it and I want to put my 105 stuff on her bike. Does anyone know if I'll be able to use her existing bottom bracket with the 105 crankset? I'm not planning on changing the BB on my bike, so i would have to order one for hers if it won't.

While we're on the topic, is there any huge reason why I should change my BB? Any real benefits of doing so? Thanks for any and all info!!!

Compatibility comes down to model(generation) #s.
Sora 3300, r3000, etc.
105 5700, 5800, etc.

List what your components are and what hers are. Without those specifics, its guessing.

It all boils down to which generation of each component line you are talking about. Current generation 9 speed Sora would probably be better than 15 year old 105 9 speed. Current generation 11 speed 105 is certainly better than even the newest Sora, but to switch over, you would have to also transplant the bottom bracket. The groupset name means very little unless you can specify which generation of those components you are talking about
You said: " While we're on the topic, is there any huge reason why I should change my BB?"
The answer is that you may have no choice if your 105 BB isn't compatible with the one on your fiance's bike and if it isn't compatible with the crank of the Ultegra groupset you plan to buy

If it's a Sora crankset, I'd assume that it'd be a direct swap with the 105. I'd be more concerned with her hub being compatible with the 11-speed cassette.

Are they both shimano cranks?

Anyway, I'd replace the BB while you're in there - they're cheap and why wouldn't you.

I'm not really sure how I check that on the sora stuff, but I'll check the 105 components when I get home. I looked last night and it turns out that she's actually got an FSA crank set, so I'm not sure the existing BB with accept the 105 set.
My current 105 BB is in good shape, so I was going to leave it on my bike, but if she's going to need one, perhaps I should upgrade mine to ultegra and give her my current one. Are there any benefits to switching out my BB to ultegra?