The CDC's Lyme disease topic page says there were 85 cases in CA in 2006, the most recent reporting year. The case rate for the entire state is 0.2 per 100,000. This is dwarfed by east coast states with case rates in the 1,000's. But personally I really try to be careful. The bull's-eye ring is present in 70-80% of cases, so that means you could easily not have it. Even for places with high incidence of infected ticks, the recommendation is that antibiotics should be started within 72 hours.
Then consider that if the disease progresses for couple of years it crosses the blood-brain barrier and becomes almost impossible to treat.
It is worth considering the fairly simple measures of tucking your pants legs into your socks (looks kind of silly) and tucking your shirt into your pants. Ticks seem to always climb UPWARDS so this way they mostly fall off before they get anywhere near skin. If you wear some light-colored clothing you can see them as they climb up your legs. If you wear nylon it is pretty slippery and they lose their grip before they get to your waist, even.
