In SF it's always best to pack for ALL possible outcomes. It could be foggy and freezing in the morning, and then clear and pleasantly sunny in the afternoon. It could be sunny with a freezing wind.
I think I would bring the dress, but just be aware you may not get a chance to wear it. It is supposed to warm up this weekend, but this past week has been FREEZING here with a lot of wind.
You will most likely be cold wearing that dress with sandals. Even when it's warmer, it's usually windy (depending where you are in the City). It just really depends on the wind factor, IMO. You definitely couldn't get away with wearing that close to the evening.
I would recommend bringing things you can layer..you'll need a jacket for evenings most likely.
ETA: Are you going to be in SF proper or somewhere else in the Bay Area?
I'll be in SF for 3 days, so I'm thinking jeans, tees and a jacket/sweater I can take on and off would be best. Then we're heading to Sonoma, so I'll save the dress and other warmer-weather clothes for that part of the trip.
I want to wear my ruffle neck JCrew top for dinner one evening, so I should try to find a jacket to go with it.
I find SF summers unbearably cold (and I'm originally from Colorado). I wouldn't rely on a super light jacket, like a blazer or similar, to keep you warm. You might need to layer a cardi under a blazer or denim jacket with a scarf. Seriously - it's that cold. Napa will be gorgeous, though. Have fun!
You also probably want to check the weather in Sonoma specifically, even though it's close to SF, the weather their can be markedly different. No matter what, Sonoma will get chilly like the city does at night. On a tangential note, if you have allergies, make sure to check the pollen count/bring meds because it can get really bad up there. Enjoy the bay!