Unless you have a magnificent computer generated film with a shit storyline, or an action packed film. There is no need to use 3D! I know it's all the rage at the moment and you can charge an extra £4 per ticket at the cinema, and make your film less interesting story wise, but it doesn't mean you can abuse this effect. Certain films are better with it and without it, pretty rubbish. Others have their own merits and should just stick to good old classic, silver screen style film making!