That's true; girth is important, too, so you can layer up inside the bag without compressing your insulation when the temps drop lower than expected.

It also depends on where and when. I don't backpack past the first week in October (the nights get too long for me at my latitude) and at least through September, even at high altitudes 15*F is the worst I've encountered. Then this year my eldest son in SW Ohio reported 0*F temps in November. It got pretty cold out here then, too, with wind chills in the single digits at my house!
May your trails be crooked, winding, lonesome, dangerous, leading to the most amazing view--E. Abbey