You can try to locate an AD outside of Cali, have the watch shipped and avoid tax. Really you might as well just take your 10%, add back your tax and get your watch. You aren't likely to score more of a discount unless you are really savvy. The most I have seen is 12%.
Depending on which DJ you are considering, the price would be between around 4500 and 7500. You can get an almost new from a reputable reseller (try bernardwatch.com) for in the neighborhood of 30% less. No visible differences...you would avoid sales tax and shipping is 30 bucks. I bought a Tag from them recently and had a wonderful experience. FYI, I own 3 Rolex Datejusts, a Tag and a few others.
You should service your watch every 5 years or so. If you send it back to Rolex, it will cost 600-700. If you find a local Rolex certified watchmaker, it could cost you half.