I prefer to adjust them with the engine hot and not running. Much easier to do, and safer. I am prone to dropping tools, and if i drop a screw driver or wrench at the fan end of the engine, it might come back up to meet me.
Once you get the hang of the order, it doesn't take but a few minutes to adjust the valves.
I have adjusted them while the engine is running, but after dropping a tool or two a time or two it's just safer for me to turn the engine off.
