yes, if you are absolutely certain a school is your top choice and you would immediately withdraw all other applications if accepted, it doesn't hurt to tell them that. But I would call the school first to see if they accept such letters. 🙂 Since schools want a high yield, they will be happy to accept you if they know you will matriculate. While the letter won't significantly increase your chances, when you are compared to a similar applicant who has no letter, they just might pick you since you show desire to attend! 😀