The ECU is tuned to be optimal at 188 degrees but that said, you can run a cooler thermo but it won't make the engine run cooler---the opening on the 160, 180, 190 are all the same once they are open/warm. So you open a little earlier---there is nothing good there--you are already well within safe temp. You can also run rich by doing that which can clog cats/foul O2. And in summer it won't stay lower anyways and in winter you wont get warm enough for heat so you wouldn't do that anyways. Run the 188---if everything else is working as it should, you are all set.
Whats going on that you would want to?