What type of lights are you installing? If they're all cans, then 3" is fine. If you are putting in fluorescents and the fixture will rest on the grid, plan on 6".
1. Add extra supports by the lights. 2. Consider anything you may have to clear, eg) duct work. 3. Keep the grid balanced - you don't want one edge to be 2 inches wide and the opposite side to be 2 feet wide.
The Armstrong site is very good. It has a materials calculator and a layout planner.