Well.. every engine runs best at a particular temperature depending on the following variables:

1) Temperature
2) Humidity
3) Fuel nitro and oil %
4) Your style (hi speed runs or blips of throttle?)

Funny thing is high end engines tend to run better when hot than cheaper engines.

Typical engines run well at 225-240 but more expensive engines run well at 250-275.

When running at WOT you should have a clean trail of smoke.

IF your engine sputters or hesitates in any RPM range/band, then you can lean it a bit.

Hope that helps.
Ram