I've always done it running after the engine is at operating temp....loosen until they clack and tighten until there's no clacking at the rocker. Sometimes if you have noisy lifters it makes things harder but I've gotten used to it. In the morning mine clack like crazy but after its run for a few minutes all gets fairly quiet. I've never had to adjust hydraulic lifters with the engine off...only solid. But as Tom said...it is better and easy on your engine, but imop as long as your slow and you don't tighten of loosen to fast with the engine running all should be fine. Either way...have fun. lol
