That is something I do often, even when I book through Marriott Platinum reservations line! By doing that, I find that I am MUCH more likely to get what I am supposed to get, and even better, much more likely to get an upgrade, if available.

(As an interesting aside, and in reference to a comment by bsb21 about Marriott associates not knowing their own rules, I had a recent stay at the New Orleans Marriott, where they specifically have an elite coordinator, who e-mails gold and platinum guests about one week before arrival to welcome them and ask if there is anything special they would like. I called her back to ask for the same room which I had the last time I was there, and to inquire about weekend breakfast coupons, both of which she confirmed. When I arrived at check-in, it was after she had gone home for the day. The associate who checked me in did assign me to the proper room, but gave me a hard time regarding the weekend breakfast coupons, and decided that only one, but not two would be sufficient. The following day I spoke with the elite coordinator, who told me that not only does she frequently have a problem with improperly trained front desk personnel, but also, sometimes with resentful personnel who "do not feel that elites should be entitled to all the extras" that they get, which is why I only got the one coupon at check-in. The elite coordinator immediately gave me another coupon for Sunday morning, and said that she would speak with the hotel manager about it, especially since she had noted in my record specifically to give me two breakfast coupons at check-in, and it is her job to handle elite customer care, as her title would indicate, elite coordinator. In this case, whether the check in associate knew the rules or not, she thought she KNEW BETTER than the rules or the printed instructions, and decided to overrule, and that it would be okay to do that. I have encountered this occasionally at other locations. AWTTW, YMMV)