I'd recommend getting a bag rated to the lowest temperature you expect to encounter most of the time (which is different that the lowest you ever expect to encounter.)

I don't expect to encounter temperatures lower than 30 (I stay home if the forecast is for lower); my winter bag is rated to 25. I carry a down jacket in winter, for camp wear, and consider it and other clothes as my "safety margin." That way, if it unexpectedly falls lower than the forecast, I can deal with it.