I guess somewhere in the high 60's with the combination of low water will usually will make me think twice about fishing. I'm like Hans, I don't see any need to kill the fish. I can find something else to do or some where else to go.
You will be OK if the stream starts out cooler in the morning and cools later in the afternoon. It's the times when the water temps are in the high 60's early in the morning and climb even higher during the day that I think are the most dangerous.
"Have no fear of perfection - you'll never reach it."