I flight instructed in a 172 that if you didn't lean it on taxi, it'd load up and you'd have to run for over a minute before you'd clean the plugs up. It all depends airplane to airplane. Now if you're in fuel injected, I recommend it when you go to 75%(Especially the ones that have the fuel-flow's with the recommended setting on them).

Now when it came to students on X-C work(really the only time you got high and steady enough), I said to do it once you've leveled off and established the pre-determined cruise power setting. Then I showed them how to fine tune it from there.