The NCAA no longer associates the First/Second Round venues with specific regions. They use a pod system which initially sounds complicated but is actually relatively simple. Imagine it like this: Each of the Sweet Sixteen teams comes out of a unique group of four teams - for instance out of the #1/#16/#8/#9 seeds in the West, one team will win and go to the Sweet Sixteen. Call this group of four teams a "pod". There are thus 16 pods. There are 8 First/Second Round venues, so each venue will host two pods. However, there is no region restriction as to where the pods from Sacramento will go on to. For instance the winners of the pods from Sacramento might be destined for the East and the Midwest, or any other combination.
This allows the NCAA to, say, put teams in the East Regional (for the Sweet Sixteen and Elite Eight games), but allow them to play closer to home in the first two rounds.
In principle, the committee will try to have the "pod leaders" (that is, the #1, #2, #3, #4 seeds) as close to home as possible. Because we have a pretty good idea of what teams could possibly be pod leaders, and because there are basically exactly four pod leaders west of the Rockies (Gonzaga, Arizona, Oregon, UCLA), and because there are two First/Second Round venues west of the Rockies (Sacramento, Salt Lake City) and because of geographical considerations, it is almost certain that the two pods in Sacramento will be led by UCLA and Oregon, and the two pods in Salt Lake City will be led by Gonzaga and Arizona...regardless of what exact seeds those teams get or what regions they are slotted into for the Sweet Sixteen and Elite Eight.