A lot depends on the rest of your gear, too. I've been in the mountains the last couple of weekends, and I've brought my 0° synthetic bag and I've been too warm. However, I've also been camping at lower altitudes, and sharing a small tent with a full fly which warms up pretty quickly and seems to stay warm.
Brilang's right, though. Expect anything, even snow. If I was car camping, I'd have a -10°
down bag in the vehicle, and if I was going backcountry for longer than a day or two, I'd carry it instead of the 0°C bag. The weight's about the same for mine, so the only real difference is the insulating values if the bag gets wet.