You could go ahead and send your LOI, it won't hurt.
Schools will usually decide pretty quickly if they want you, or if they don't, or if they would like you but aren't sure you want them, or if you're qualified like many others are and the most interested should get in. These last two are the waitlisted, and this is usually why people send LOI's, to let the school know their intentions so an acceptance won't be lost on someone who won't end up there either way.
So unless they've decided to admit you already, or to reject you, an LOI won't hurt, and they'll keep it on file if you're waitlisted for consideration. It is not a golden ticket to write an LOI, because they still have you ranked as far as who they want to admit first off the waitlist (depending on the school and your performance). But go ahead and send it if you do intend to go there if admitted.