This topic has come up a couple of times in the past. Most folks seem to opt for lower coolant temps and the "extra pad" it allows before overheating. However, it is useful to consider that the internal engine components are designed to operate at a specific stabilized temperature. To that end, the parts are machined so that when the parts heat up during combustion, the thermal expansion of the components achieved at stabilized temperature establishes the proper clearances between moving parts. This web site:
http://216.239.33.100/search?q=cache...hl=en&ie=UTF-8
discusses, among other things, the relationship between stabilized coolant temperature and cylinder wall wear. Notice that at 70 deg C (about 158 deg F) the cylinder wear is significantly higher than at 85 deg C (about 185 deg F). Admittedly, the study is on tractor engines, but the principle is the same. Most of the earlier engines were designed to run at about 185 deg F coolant temperatures, and the later ones run at close to 200 deg F (to reduce emissions). Running at significantly lower stabilized temperatures could result in accelerated cylinder and ring wear, due to increased mechanical clearances and dilution/contamination of the lubricating
oil with fuel blow-by. IMHO, it's better to get the cooling system running properly and at a constant temperature than to pad the temperature margin running lower than normal temperatures.