Jon, I’d say it depends ENTIRELY on your personal sleep temperature. You’ll get a lot of people telling you to buy the 20 degree bag for the whole trail AND use a liner, you’ll get a few telling you to get a 15 degree or warmer, and a few telling you to get a 25-32F bag. You’ll get a few that tell you to carry two bags – one for the desert and norcal/oregon and a separate, warmer bag for the Sierras and Washington, if you can afford it.
But ultimately, it depends on you. Know yourself – take a 20 degree bag for a test run if at all possible, or preferably base it on previous experiences. I started with a 15 (really more like a 20) degree bag and sweated my FACE off. I sweat so bad all the time. First chance I got, I bought a Western Mountaineering Summerlite – a 32F bag. I discovered that the “common wisdom” about a 20 degree bag for the whole trail did not apply to me. I weathered several rainstorms in Washington where I woke up to a frozen rainfly (frozen SOLID, with ice caked on) and had slept fine in my 32F Summerlite inside my tent.
Of course, I’m definitely on the warm end of the spectrum. Basically, get out there and try some bags if you can. I’d say a 25F bag with a liner would be perfect for the entire trail for an average sleeper, or 20F with no liner. But that’s only for an average sleeper, which you may or may not be.
Sorry to be so verbose, this is one of my favorite topics. 
Joker