Your cruising along at 6000 feet for example in a standard Cessna 172 with a fixed pitch propeller, you've leaned the mixture. You decide to climb to 8000 feet at full power.
A C172 climbing from 6000 ft ???
Are you kidding me ?
Seriously though, I've always erred on the side of caution and leaned above 5000 ft until the engine is running smoothly again.
Leaning for max RPM is dangerous (IMO) because now you're running the engine at peak egt (albeit at low power, after all how much power can a normally aspirated O300 or O320 produce at these DA's?) .
Running excess rich may be considered safe, but NOT when it's likely to foul plugs and wash away lubricating oil from the upper cylinder area.
As for auto-rich....I've heard of it but not since the DC-3 era.
Certainly, my O-360 doesn't have it. (just as well too!)