there is a difference in being a coward and havin a strong feeling of self preservation. cowards let fear dictate their actions, sidious would rather run than fight yoda becauase he might lose, his entire lifes work was being realised and he didnt want to risk it needlessly on the outcome of a single fight. dooku had the plans to the death star and had to get them to his master, an i he kept fighting hed likley have died. your not a coward if you know your capabilities and act accordingly. you an idiot if you let your pride get the better of you and you stand your gound knowing your going to die for no reason other than your too afraid to run.
grievous wasnt a sith, and like dooku said, if he doesnt have the advantage of intimidation and fear he would be well advised to withdraw. which he does. he didnt become one of the greatest jedi killers in the galaxy by standing his ground to the likes of mace or yoda. he had a job to do, and pride was something he checked at the door when need be.