I live in the San Francisco Bay Area. In the summer, as it almost never rains here, I end up watering plants in the backyard every 2 to 3 days. In the winter, it rains more regularly. However, when it doesn't rain for periods of a few weeks in a row, should I then be going back to watering, at least for non drought-tolerant plants? Or are the plants in semi-hibernation, making watering in the winter unnecessary?
Our climate is different, but I would let the soil tell me - if it's dry below the top half-inch or so, I'd water thoroughly. Don't just sprinkle - it encourages shallow root growth, when you want them deep instead.
I'd expect water needs to be reduced in the winter, but not gone. It will vary greatly from plant to plant.
As @Ed says, let the soil tell you - also let the plants tell you: plants do vary (a cactus will be more drought tolerant than a magnolia, for example).
Also, try to plant local natives - I've spent a few months in the Bay Area and there's a lot of wild plants around there. They survive just fine without any human-produced irrigation.