Overheating at idle has a lot more to do with air flow than the thermostat, although I suppose a lower temp would give you a little more cushion timewise..

My PV used to overheat rather rapidly sitting still at idle in warm/hot weather. The only way to combat it was to rev slightly and/or turn on the heater (not fun when it is already broiling out). I fixed it by adding a thermostatically controlled pusher fan in front of the radiator. The problem was just that the puny little 4 bladed fan on the water pump just didn't do much of anything at idle - sitting a few inches back and without a shroud. At higher rpms it actually moved some air, but if you were in gear the air pressure on the grille does most of the air moving anyhow.