I think of the sensor in my camera the same way that I think of the screen on my big TV.
The boundaries of the sensor or the screen don't determine the composition of the content.
I can watch Casablanca at a 1.33:1 ratio or Forbidden Planet at 2.35:1 on my set. The content is not required to fill all of the space on the screen all of the time.
The most famous medium format camera of all time is the Hasselblad which shot 2 1/4" square images. It was the king of advertising and magazine photography. It went to the moon. I'll bet 95% of all commercially reproduced Hasselblad images were never printed full frame. In fact, even today, most widescreen movies are shot on a full 35mm frame and cropped for theatrical presentation.
Don't let your vision be determined by the shape of your sensor or the popular "snapshot" printing format. Yes, 4x6" is the most popular format for small printing. But 11x14", a common large print format, is more closely matched to the 4x3 aspect ratio. 4x3 also fits 8x10" better than the sensor shape found in most APS-c DSLRs...but these considerations should be secondary. There is no one ideal aspect ratio...for prints or movies.
The picture should be composed in way that best suits the material and your vision of that material.
If you are doing a run of pictures for cousin Ed's Birthday party and you plan to make a bunch of cheap prints, by all means, leave extra space so that people don't get their heads cut of in 4x6" prints.
Just don't use the snapshot format as your guide for serious composition.
