Since most of the legionnaires' bacteria can also reside in your aerators, shower heads, etc... in your system, keeping the water at 140 degrees at the heater is not guarantee that they will be killed at these areas as the water will cool down in transit. Further, adding in a mixing valve to cool down 140 degree to 120 degree or lower at the shower head or tub spout would defeat the 140* water. In short, I'd set my heater to 120* and reduce heating costs, risk of scalding. The pdf I referenced said 120* will kill 90% of bacteria in a few hours exposure (a bit slower than the 140* which also is about 90% kill rate) so you aren't losing much.