I have not fished very many places this summer due to the high water temps. My favorite stream for this fishing has not been over 64 degrees any morning I have been out. Another favorite stream has been too warm to fish, since it goes over 72 degrees regularly late in the day. A third one tops out near 70 degrees on the real hot days, so I have fished it only once or twice when we have had one of our few cold nights.
I think if a stream is cool in the morning and tops out at less than 70 degrees late in the day, you are ethical to fish it during the morning.
This is much different from the way guys thought when I started. They sought spring holes on marginal streams; and when trout congregated there during the hot weather, they tried to catch them. They made fine catches and sent many trout to their doom in the pre-catch-and-release days. If you read James Bashline's book about night fishing, these were the kinds of holes that figured prominently in the night fishing adventures of that time.
If you read the Shields/Harvey collaboration about trout fishing, George Harvey caught his largest open water PA trout during a night fishing adventure in one of these places where a number of big fish had lined up. As I recall, it weighed 8 1/2 pounds.
Pretty interesting topic -- one that involves a number of thoughts about the ethics of trout fishing.