While they might not have a stated preference, when I ran the numbers two years ago, UCSF had a 4.32% IS admit rate and 2.55% OOS while USC had 7.20% and 2.67% and Stanford had 2.76% and 2.43% (the only one I saw that wasn't materially different). I didn't do the analysis for UCLA, which is the only other school I might have looked at. And, this is even with IS yields being way higher than OOS.
If there was no preference, how would one explain UCSF offering interviews to 261/3517 IS as compared to 215/4243 OOS in order to enroll a class that ends up being 115 IS and only 34 OOS? I think this evidences a preference at every stage of the process (IIs and As). To me, it look like they do what they need to do to get a 75% IS enrollment, which is very consistent with the mission of a state school, even a T10. If this weren't so, why don't all of the T10 have a 75% CA enrollment??
(I know, I know, all schools attract candidates due to regional preferences, but still, Harvard isn't 75% MA or New England, Penn isn't 75% PA, etc.! THOSE private schools don't have meaningful IS preferences.)
I became so disillusioned by the OOS numbers that I decided to look no further, but my observation is that the UCs definitely have a preference, even if they don't state it!
There is no other way to account for such a divergence in favor of one state (coincidentally, CA) over the other 49, as though CA has more, better qualified candidates than the other 49 states combined.