I hate to but I've got to throw my opposing .02 cents in here. I run a 82'C thermo in my 95 E320 and it will run to as high as 90, in the middle of the summer, running the air, stuck in traffic, and around 80 in the winter. We are lucky enough to have to pay $25 each year for an emission check here in Atlanta and last year during the cooler months my car blew as clean as a new one running at 80'C. I saw no decrease in MPG, my only intention was to cool it down a bit during the hot summer here because 100-105 was not an uncommon sight on my temp. gauge prior to me installing the 82'.
And I'm sorry but I don't care what Mercedes says, 105 -120'C "with a properly working cooling system" is just too damn hot for any car. So the 82' thermo changed all of that with no ill side effects.
Now if you just want to run your car hotter than go for it, but if your ok with 80, than I can assure you the car is not running rich and it will not hurt your gas mileage.
Now would I go below 80? Probably not because I don't want to go too far down on the temp. scale to where the engine doesn't fully warm up and run clean. It's a double edged sword because too cold will cause increased wear and your gas mileage/emissions will both suffer. Too hot and your timing gets retarded, performance suffers, your cooling system is stressed out, oil will degrade faster, along with an unnecessary baking of everything in your engine compartment.
Ok, now that I've thrown my .02 cents in here let the flames begin!!!