From my esperience last year, I am going to guess that Roger will always practice on the 4 courts closest to Ashe stadium. Last year those courts were mostly blocked by bushes, trees, and fence covers. There is one practice court that has bleachers and is much more open that Andy practiced on once and there was nearly a stampede when he came off the court.
Most players will have a warm up hit anywhere from 1 - 3 hours before their match (depends on the player and the time of the match) so that might give you an idea of what time to head over to the practice courts to look for him. I would not go out to the back courts looking for Fed, I doubt he will ever be there. If you are going on the first day, be prepared to be a part of a very large, poorly managed crowd.
Wear plenty of sunscreen and comfortable shoes, and let us know out it works out for you.