Both. I would request when you reserve, AND request when you check in. What's the worst that can happen? You don't get your request.
I've always limited my requests, because I think the more things you request the more chance you have to be disappointed. Also, the more specific your requests are, the less likely they are to be fulfilled.
We actually never really worry much about requests, because we use our villa to sleep in. About the only things we've really asked for have been first floor for medical reasons when Grandma accompanied us, and non-smoking for medical reasons because of DD's asthma. Non-smoking is now a non-issue (until we get a room that has been smoked in), and we've always gotten our other rare medical requests.
On arrival, I've looked at where they were assigning us and tried to improve things, but frankly have never been very successful doing that.
I think specific view and location requests would be much more important for folks who are spending large amounts of time in their villa, or who are not going to the parks at all.