I just experimented saving with or without the aspect box checked and it does not seem to change anything as far as what is saved. It just changes how it is displayed in MP3Tag. This was also the case in Foobar. If I check the correct aspect ratio, it just changes what is displayed, IF it did not have the correct aspect ratio when saved. If the file had the correct aspect ratio, then there was no change, as would be expected.
So I think I was off-base, and MP3Tag just saves the image you store, as is, and then just changes the way it is displayed in the program, for user preference.
Therefore, I guess the wise strategy is to make whatever changes you want before saving the images and save it with the correct aspect ratio. That way you will not be affected by the check box.