Below is a 3:2 image from a digital SLR, followed by examples of how it looks when stretched/squished to other common ratios.
3:2 (500 x 333)
Most 35mms |
Screen Resolutions: None |
|
8:5 (500 x 313) |
Screen Resolutions: 1280 x 800 |
|
5:3 (500 x 300) |
Screen Resolutions: 1280 x 768 |
|
16:9 (500 x 281)
High-Def   TV; |
Screen Resolutions: 1280 x 720 |
|
4:3 (500 x 375)
Most |
Screen Resolutions:
640 x 480 |
|
5:4 (500 x 400) |
Screen Resolutions: 1280 x 1024 |
There is no "best way" to deal with this lack of uniformity among mediums. But the good news is that unless you're designing wallpaper or making prints, it's not going to be a big problem. In those cases, you have two options. Let the image be stretched/squished out of proportion, or crop it to the desired ratio. Both of course alter the appearance of the image so which way you go is a matter or personal preference.