I have sometimes pushed my 30F down quilt to the moderate conditions in the sierra, e.g. lows typically not below 0F. The first was a surprise cold front in early fall when we weren't that high. Dropped to 12F when we were expecting a low of 30F. i wasn't comfortable at the coldest part of the night, but was warm enough to be safe and to eventually get back to sleep. Since then I have occasional taken my quilt on winter trip when I someone needed to borrow my winter bag (WM Versalite which I am toasty in at 0F with no clothing).
For me it make sure enough padding, use my bivy to keep side drafts out, use a a warm hat + a down baklava, wear heavy base layer, and high loft (down and/or synthetic jacket and pants).
That said, while I tend to take less insulation that many people I know on 3-season trips, I really do enjoy a warm bag on my winter trips.