I used my 30 degree bag until Daleville, VA. then I swapped to a 15 degree bag. It was none too soon as there was a very cold, rainy week at that time in mid October, with a bit of snow as well. If I had waited until Pearisburg to swap out my bag, I would have been pretty cold for a few days.
I did hike with a couple of other SOBOs who used 20 degree Cat's Meows the whole way. A 20 degree bag is a nice compromise for those using a single bag for a SOBO hike.