That would become an issue in actually creating the texture.
For me, I work on aircraft, and rely on having 3-views to create top, side and front (and sometimes bottom) views of aircraft. This normally resolves many issues (at least from an external standpoint). For things like landing gear wells, weapon bays, and cockpit and crew spaces, I have to rely on photo documentation, which is almost never perfectly shot from the right angle to make creation of textures easy. In those cases, one tests one's creative skills at creating textures....
About aspect ratio... once you're working with the Texture Coordinate Editor, you can move things around at a vertex or surface level, so this eliminates a lot of problems with aspect ratio. If your texture has an area or object that is correctly sized, you can identify the surfaces of the model, and move the vertices to fit the area. Provided both are based on the same dimensions and ratios, it usually looks great.