We did the California wine country thing around 4 years ago - a lot of fun! Not sure how many different wineries you're going to visit, but they are often quite different from each other. We preferred the smaller vineyards to the big commercial ones - the staff was much friendlier and frequently the wine was better. I joined one wine club on the tour and their wine was consistently excellent.
As someone mentioned, eat a good meal before going so the wine doesn't affect you as much. I'm not sure I ever spit anything out into the spittoon - pretty much drank everything set in front of me. What a wino! But it is a very small portion per each wine.
We dressed very casual. The only time we were a bit dressy was for a fancy-pants dinner we had one evening. Have fun!