Yeah I know the smart thing to do is err on the side of caution, but what's the fun in that

I have not spent enough nights out in differnet conditions yet to know for sure how to gage my comfort level sleeping in different temperatures. I do know my Kelty 20F bag is fine in the lower 30's. I also know the Kelty bag is perfectly comfortable in a very leaky tent in a monsoon at around 60F, although the situation was somewhat disconcerting!

Also I have never been far enough out or put in a situation where my equipment choices could comprimise safety, but your point about situations changing is a good one.

I think the smart thing to do is go for the 30 degree quilt, it should be more versatile than the 40 degree, and worst case layer up make the best of it!