Personally I don't think Grievous was much of a coward at all.
He retreats from the battle with Anakin and Kenobi because the ship was falling out of orbit. That's not cowardly, that's smart.
On Utapau he fights Kenobi one-on-one, even though there's thousands of droids that he could have ordered to do the dirty work for him.
Even after losing two hands, he's still ready to continue the fight. Kenobi does the cheap thing with a Force push. Only then does Grievous run. And why not? There's a clone army breathing down his neck, he's lost his weapons and is facing an armed opponent. Again that's not cowardly, that's smart.
Plus, when Kenobi catches up they fight again. If Grievous was a coward, wouldn't he have just surrundered at that point?