I dont run a therostat in my 500 caddy as it wants to heat at idle(they have a really lazy water pump) its not a cold weather car and it seems to help at low speed cruise or in the line at the strip idling.
In my area you have to run a thermostat on the street no matter what in the winter , We also have those funny plug ins that hang out of the hood block heaters battery heaters oilpan warmers car warmers etc