A lot depends on whether you're a warm or a cold sleeper. I get COLD really easily, and my WM 20* bag was rather marginal on a few frosty nights in the Wind Rivers even with extra layers on, although I very much doubt that it actually got below 20*. There's a lot of individual difference here, and you'll have to experiment a bit to see what works for you.

My problem is that if I'm wearing all my wraps to bed, when I get up (which I have to do several times during the night, unfortunately) I get really cold while out of the bag, so I'm shivering for at least half an hour when I get back in before I can fall asleep. The same thing happens in the morning, which means I'm liable to stay in the bag until the sun hits the tent and end up getting a very late start, YMMV!

If you can be comfortable on below-freezing nights in a 30* bag with extra layers, more power to you--I envy you! It certainly didn't work for me.

The best thing about my 20* WM bag (Ultralight Super) is that wonderful draft collar, which I can snug around my neck--it provides the extra warmth I need.