Like everyone else has said there is no concrete rule for this.
However I will attempt to answer the 'take standard wide picture which is compatible with wide TVs, monitors, etc.'
I find when taking photos, you shoot to fill the frame, and may not necessarily consider changing the aspect ratio until after. An example of this is your picture is 2:3. Instagram (as an example) used to only allow images which were 1:1. This will result in a heavy crop to upload it on there and part of the image could be lost.
A resolution to this is upload the image with an outer border or frame, so keep the image as is, but pad it to the specified ratio of length/height to fit the output dimension required. If you're more graphically inclined, you could do it with borders etc to make it look better then two black bars either side for example.
Create these various 'frames' for common dimensions you wish to use, so 16:9 for widescreen television or 16:10 for monitors.
it may not be the most glamourous approach but does result in the same image 'compatible' without distortion/crop/widened/shortened on various devices.