in my limited experience
I have trouble experimenting in water less than 40 degrees. I have to drive 2.5 hours, so the urge to switch back to chuckin tactics are very strong especially if I am on a day trip and I have "ONE OF THOSE MORNINGS". If I have an exceptional day and the steelies are turned on, I go to swinging flies reguardless of the water temp. Sometimes even the chuckin methods yield a hit a day though so it's a long drive for no hook-ups. Better weather though and I will swing all day. The best days I have had were in the 42 to 50 dgree water temp either in the fall or spring. And if there are drop-backs in the river, they'll hit any swung fly.