let the director decide! unless the emphasis is crucial, then it would be italics or CAPS
Yes, the director has the ultimate word, whether the actor puts emphasis on the word or not. However, it doesn’t hurt to italic words that truly deserve an emphasis, if anything it just helps create the visual image in the reader’s mind. It’s not like it gets in the way of the story like including camera directions in the action does, or a crap load of parentheticals, because it’s already a part of the dialogue.
If the director decides not to use your emphasis on certain words, and wants his actors to deliver the line a little bit differently, trust me that’s the NOT the worst thing that can happen to your script. They can take out scenes, add scenes, change scenes, change dialogue completely, change the chronological order, add characters, remove characters… change characters, There can be a lot more to be pissed off over, trust me.