A It depends entirely what your intentions are but here are the general format uses to help you out.

 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.
 4:3 NTSC format: PC software, Commercial Presentations (if your projector is SVGA or XGA native resolution), Classroom Training, and Gaming.
 16:9 HDTV format: Home Theater Movies and Gaming
 16:10 format: PC software, video games, video editing, presentations from notebooks with WXGA or WUGA native output.
 2.35:1 Cinemascope format: Strictly Home Theater movies.

A For the best possible results, an anamorphic lens is recommended. However, it is possible to get a 2.35:1 screen format as long as you can live with the "black bars" created when a mismatched 16:9 native aspect ratio projector is set up to fit a wider projection screen format. It is simply a matter of personal preference.