ambush should be ur first priority for not only the duration but the speed increase, So u should be stealth long before the sound is a factor unless cornered in which case gettin away is more important than if they now you there other wise mean u have them cornered in which case if they hear it its to late ambush is not indefinate. so as said above wait till last min and use it while in the fog and i do not believe the shrooms will let them see you but teemo can see them on the map if they are paying attention they will se you coming but never hurts to take the long way around when fighting peple who can see you

sorry if its hard to understand im not the best at putting my thoughts into words with a keyboard