I've been going Training at events in California all this year and have yet to stud for anything but show jumping at May Galway (beside the point but whatever). However I haven't been to Ram Tap, so I don't know about that.
I do know that many people stud for Shepherd Ranch, although I was not one of them.
So far, I know that many people stud for the 3* at Twin, the Advanced at Woodside, 3* at Galway, but beyond that I haven't heard of anyone using studs. Could be wrong, I'm sure there are some, but not that I know of.
I used to stud on any grass course at Training, which would include Eventful Acres and Shepherd Ranch. No need to stud on the dirt courses at Woodside and Ram Tap. I haven't run Twin Rivers but I can't imagine needing them there either.
The one exception might be Woodside if conditions were very muddy, because there are significant slopes there.
My mare was pretty particular about her footing, so I sometimes used studs on grass for her at Novice as well. Grass in California is often very slick.
If you are allergic to a thing, it is best not to put that thing in your mouth, particularly if the thing is cats. - Lemony Snicket
I would recommend having your horse's shoes drilled for studs anytime you're competing on turf. You never know when you'll be riding on a dewy morning, or on summer-slick dry ground. With that said, the majority of California events are on groomed tracks, so it's rare to pull out the stud kit.
Eventful acres for sure, year round. Its on turf and training normally runs in the morning. So even in the warmer months the grass can get slick.
Ram Tap you almost never need studs. I ran Ram Tap in Feb and it RAAAAINED. The XC course looked like cake batter it was so sloppy but a lot of people (including myself) ran without studs and it was no problem. One of the BNTs even commented on how nice the footing stayed.
Woodside - If the ground is wet yes, it doesnt stay nice like Ram Tap it gets slippery.