Amumu is a disgusting beast on the grubs, you can probably mid max his clear speed on them a little bit as well switching focus between them to keep then consistently the same hp. But I'm not considering if u need to factor in bailing out and keeping your HP higher. NASTY
Always get the leash. The extra damage gets your clear done quicker and sets you up to do something earlier if possible. If your lane isnt coming to leash they are trolling you.