Depends. You usually try to "keep the shape" - for organic models, your UVs would look somewhat similar to a skin rug, for machine parts your UVs would be somewhat rectangular.
For cars, your UVs of the sides would just look like someone took a photo directly from the side, without stretching the silhouette - very useful for paintjobs.
Keep in mind that painting patterns or "geometry-aligned" shapes will be much easier if you straighten your UVs. Also, baked stuff will look better because there won't be as much aliasing (or blurriness due to antialiasing).