Having just returned home from SF, I'll throw my $.02 in here. You can walk across the bridge at certain times of the day - I think it's 6am to 5pm or there abouts. This will not allow you the opportunity to walk the bridge for any night photography.
I did most of my shots from the Marine Center near the wharf or from Crissy field. While I did not get any slow SS for traffic, I did get sveral from both sites that are more than adequate for a first time to SF. Both areas can be walked to from the wharf and both walks are pretty safe.
As for transportation, you can take a cab from your hotel to the field or across the bridge. Crossing the bridge I would think will be expensive as you have to pay for the ride over there and for his time waiting while you get pics. If this is what you really want to do, rent a car. However, if you are willing to stay on the south end of the bridge and do some planning and walking, then no car is needed. I thought parking was pretty expensive, but I don't live in a big city, so I have nothing to really compare it to. Not to mention, where you park, you'll end u having to walk anyway. Also, walking provides some great views of the city that you might miss if you ride all over the place.
Just my opinions from a my recent trip sp take it for what it's worth to you.