A. It depends entirely what your intentions are but here are the general format uses to help you out.
1) 1:1 format: If you use traditional Slideshow/OHP (Overhead Projector), Document Camera, and commercial projector presentations. These are most commonly used in schools, government facilities, and houses of worship.
2) 4:3 NTSC format: PC software, Commercial Presentations (if your projector is SVGA or XGA native resolution), Classroom Training, and Gaming.
3) 16:9 HDTV format: Home Theater Movies and Gaming
4) 16:10 format: PC software, video games, video editing, presentations from notebooks with WXGA or WUGA native output.
5) 2.35:1 Cinemascope format: Strictly Home Theater movies.