I hike primarily in the Southeast, and my advice would be to buy a bag rated at the temperatures you expect to be in the most. This is probably something you will not figure out until you get out a few times. When I started hiking my first inclination was to buy a 0 or -10 deg bag. Now 7 years later, the coldest temperature I have ever even been out in was 12 deg, and I don't plan to do that again. So, I would recommend maybe a good 15 deg bag as a starting point, and then if you get into hiking you'll probably want to get additional bags for different scenarios.
Note that how you sleep (i.e. are you a cold or warm sleeper) as well as your shelter can make a big difference in your comfort.
Just for your info I have 3 WM bags: a Caribou (35F), an Ultralite (20F), and a Versalite (10F). I use the Caribou about 2/3 of the time, and just wear clothing inside of it if it's going to be cold. I use the Versalite the rest of the time. The Ultralite will probably get sold. I have also hiked out West a few times in the summer (Southern Utah and Sierras), and the Caribou was sufficient for these trips.
Edited by Berserker (02/04/09 01:28 PM)