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Yeah, varies depending upon engine design and usage. Typically keeping it within design-envelope of temps would be best.
My '08 race-bike's been through 3-years of racing with +40 track-days per year with factory radiator with thermostat and fan removed for more flow. Warm-up takes longer on cold-days, but it does fine on +110F days at constant full-throttle. I do occasionally see yellow warning-light go off at 230F. I do give it 5-minutes cruising at low-loads after each session to cool off.
Ideally you want to run as high-temperature as materials can withstand. That's to reduce delta-T between combustion-chamber and surroundings which rob heat that could be used to expand combustion-gasses that push down piston. The more you cool combustion, the less pressure you have pushing down piston. That's benefit of thermal coatings on pistons tops, valves and cylinder heads, too reduce heat transferred away and keep it in combustion chamber.
Honda's actually been experimenting with ceramic-based engines that doesn't use any coolant at all and runs at +1000 degrees. No heat-loss to coolant jacket and they're getting higher power and efficiency.
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