You will run slightly leaner at full engine temps but you want your engine to run there anyway. Running cold is just as bad as running hot. The parts that can be damaged are different and it happens in a different way but it's still damage. Running too hot will cause rapid damage while running too cold, and therefore too rich, will happen at a slower rate. However, running too cold and too rich at the same time will quickly damage engine bearings from fuel dilution.
Our engines are designed to operate effectively at or around 80 - 85 C. Constant running at or over 100 C will warp the cylinder head. You are probably safe at 75 C to 90 C but I can't recommend operating much outside of those parameters. A 79 C thermostat will often give you around 82 C which is right in the sweet spot. The main thing to watch for is large spikes in engine temps which can be very harmful to your cylinder head. If you find this happening while waiting for a stop light you probably have a cooling problem. This is not normal and should be addressed without delay. I also highly recommend water wetter from my own personal experience.
