Last winter I was getting pretty bad pain in my knees during the coldest periods. My knees seem to take longer to warm up in the cold. If I'm going on a short ride (most of my rides are as I'm a commuter), I like the lower ratio so that I avoid tendonitis.

During the summer (maybe it's psychosomatic), my legs seem to be less sensitive and appear to handle a higher ratio. Braingel's point is pretty good too, as a general safety precaution.