I routinely get free upgrades when using hertz. I always choose the smallest car which they never have available and they will offer you the next size up free of charge.A colleague of mine always seems to blag a "free" upgrade just in case that's of any use to you and you want to try it.
But if you are going out of the city e.g. to Yosemite or Napa Valley, definitely get a car. If you book online ahead of the trip, you will get a much better price. I usually use Hertz, they are quite good. Never got a free upgrade but usually they would offer it for a small markup fee which works out less expensive than if you got the larger car in the first place.
I'd also recommend getting at least a 2 litre engine, not very eco friendly but there are a lot of hills in Yosemite!