I look for lap times when adjusting the settings and gearing to find the best combination. Whatever gives me the best lap times over the duration of a race is the best setting. I only look at temperature to make sure I don't harm the motor. Sometimes I get the best results at a lower temperature than I'd expect, sometimes higher. It depends on the track and layout.
I only use the 74C number as a maximum safe temperature (that is my opinion, I know some people who do run slightly higher temps)