If you get put on the waitlist or around Feb-March-April, I'd send a letter of intent. It's to essentially say, "Your school is my #1 choice. I'd like nothing better than to spend the next 4 years at your school. If admitted, I guarantee I will attend." At least, that's my understanding.
Whether it makes a difference or not probably depends on the school and how they run the admissions process. I actually submitted 2 letters of intent, spaced by about a month and half to two months, late in the cycle (March, April I believe). I got in...but I don't know if they had anything to do with it.
Initially I thought, "What if I make them mad by sending too many letters of intent?" Then I thought, "This is my future. I love the school and I'd rather be there than anywhere else." Moral of the story: make your feelings known to them! Good luck and blessings!