To ensure video content is not stretched inappropriately during playback you should ensure your source content is encoded with the correct display aspect ratio and square pixels.
Ensuring your aspect ratio is the same as your playback locations will prevent black bars from being displayed on your content during playback. While different devices might have different aspect ratios, you should try to make the content match as closely as possible.
In addition to display aspect ratio, you must also ensure your content has a 1-to-1 pixel aspect ratio — your pixels must be as tall as they are wide. This is another problem that is uncommon with completely digital footage but is often found with content that was transferred from older mediums such as DVDs, film, or anything using the PAL format that is common in Europe.
Because most devices are unaware of the pixel aspect ratio of played content, they simply display them as square. This means your content is displayed skinnier than appropriate so things will appear taller and skinnier than they did in the original footage.
While this issue can be corrected in the transcoding process, it is best to work with source material that has been corrected as early in the production process as possible.