I agree. I came to the conclusion that to trade butterfly intelligently, I have to be able to analyze the set up. To me the most important factor is my opinion on what the underlying will do during the period. If you think you can get that from the parameters you mentioned, you are way ahead of me.
Haha Ironchef.. I can have at best a vague idea of where the underlying will be trading, however if I find a combination with risk:reward of 1:10 on a strike that I think unlikely (but statistically possible), I will still put it on.