The point about individual cold tolerance is well taken but most people are better served with a sleeping bag a bit too warm than a bit too cold.
On the other hand I think one reason people get cold is that they use too warm of a bag zipped all the way up too early in the night. They sweat and then get cold as the temperature drops through the night. The warmer bag can work, but you need to take a little more care in regulating it as the temperature drops. I find I am a lot more comfortable if I have just enough bag.
My strategy is to be comfy in my bag in normal overnight temperatures and OK with all my warm clothing items either worn or draped over me for a record low. I have rarely not seen frost at least some point during any of my long trips or backpacking trips and the odd colder night is not all that rare, but my Phantom 45 (EN 40) has worked out extremely well for me.