Do it when you make your reservation. You can do it later, but they do not send you a form. If you do it later than when you first book, you probably won't get another written confirmation with the request on it unless you ask for one.
You can actually do it after making your reservation by just sending an e-mail to MS from the web site. I did this the other day, and got a confirming e-mail back within 24 hours saying they had added my request to my reservation (and of course saying "these are just requests, not guarantees)! That worked really well, IMO, and I didn't have to wait on the phone.
If you know what you want when you book just go ahead and request it then. The problem with most items is that they are only considered requests and are not guaranteed.
As others have said, its best to do it when you make your reservation. Doing it later just makes extra work for you and DVC. Once you've stayed at a particular DVC resort a time or two, you'll know already what you want to request for that property.
I don't usually make requests, but when I used to, I always made them at the time of booking. I figured you had a better chance gettin it in at 11 months out. Of course, that was when my MAIN request was non-smoking for medical reasons. Now that all are non-smoking, I don't bother making a request. I just take what I get, and I'm usually happy with it.