Letters of rec are a part of the secondary application, they'll ask for them. Some of them even have special pages that the want filled out by the person who is recommending you (I've found that these forms don't have to be filled out). Remember, supposedly the primary application is evaluated and then a secondary is sent if they like what they see. You would be waisting your time and theirs if they decided not to send you a secondary but you had already sent in your letters.