Regardless of numerical ratings, staying warm in a sleeping bag has a lot to do with things other than the sleeping bag brand and model. First, the bag has to fit you, too large and you get drafts, too small and you squish the loft. You have to be willing to use the hood system to stay warm at the lowest rated temperature. If the hood and draft collar are poorly designed or do not fit your body type, then you do not get maximum warmth. Women and men are shaped differently - large shoulders vs small shoulders make a difference on how the hood fits. And as others have said, for warm nights, a full zipper vs half or 3/4 zipper makes a difference.

Non-sleeping bag issues -- being hydrated, a good lasting high fat/protein meal before bed, getting into the sleeping bag when you are warm vs chilled, quality of sleeping pad, other clothing, humidity inside the tent, wind if sleeping under a tarp only, on and on!!

I would rather error a bit on the "too warm" side. I have never liked regarding my extra clothing as part of my sleeping system. Usually cases where I am dangerously cold, are cases where I have had to resort to using my insulating clothing to stay warm during backpacking (getting caught in a storm on a pass, for example) in really poor conditions, where sometimes they have gotten wet as a result. My sleeping bag is always my "last resort" - kept dry. In fact, in severe storms when I worry about tent failure, I pack up my sleeping bag in a waterproof bag and put on all clothes and sit inside the tent on my sleeping pad, waiting out the storm. Also, ounce-for-ounce, a slightly colder rated sleeping bag is a more efficient way to add warmth than added clothing.

I have always had a 0-5 degree bag, weighing just under 3 pounds total. I am much more minimal on clothing. I now use a WM Super Antelope (very old bag, 750 down). When it is hot, I simply throw it over me as a blanket. If I could afford many bags I certainly would buy a 30-degree bag. But for a "do-it-all" bag, I would not consider anything rated less than 10-degres. (I do mainly high altitude, Sierra and Wind Rivers).