Originally Posted by scoobertron
Unfortunately, with recent firmwares, the cover generation is done on the fly, rather than all at once when books are added.
I just checked the impact of this by loading some books onto the kobo, opening the library and browsing a couple of pages with the margins set to 0. The covers generated properly. Once I opened a book and set the margins to a non-0 amount, any new covers generated had margins added, and the cover image was squeezed into the space between the margins - ruining the aspect ratio.