Gary, the most important reason for using a mixing or tempering valve is to prevent scalding you or your family. 150* can cause 1st degree burns, 180* can cause 2nd degree burns.
I have a mixing valve on the output of my domestic water heater, when the coal boiler is in operation, the water in the tank is about 150*, so the output is mixed with some cold water to temper it. So far it is working pretty well, I do soften this water, so I hope it will last a long time.
For safety-sake, I'd install one, take a look on Ebay, you may find a good deal, I did.