I've been in these discussions before and did some research on the topic. I'm not a biologist and memory serves me only so well, but one thing to keep in mind when considering how water temps effect trout is the effect of fluctuation of temps over time. Most streams will fluctuate 10-12 degrees or more in a 24 hour period. If you find water temps peaking at 4:00-6:00 PM at 72, chances are from 800 PM through 10 AM, they have been under 68, a very comfortable temperature for trout. Several hours in the 70s, even if the peak is mid-70s probably causes very little stress, except if there is a low dissolved oxygen issue as well (oxygen dissolves easier in cool water). Then again, if the water is above 72 degrees for 16 hours and only drops slightly below 70 for a few, even if you fish at the coolest hour of the day, you are going to find stressed fish.
Similarly, have the trout been under temperature stresses for several days or weeks, or is this the first day water temps peaked above 70? If it is the first day they got above 70, fishing for them even with a water temp at 74 will probably not do a lot of harm. Fish that are stressed will not magically become healthy when the water breaks back below 70, though recovery will begin. Likewise, fish that are healthy will not be weak and vulnerable as soon as water temps break 72, but they will begin feeling some temperqature stress. In summary, it helps to consider and factor into your decision the amount of time that temperatures stay above the optimal.
Please bear in mind that I have no idea what I am talking about.