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!