Peter Spier wrote:
If you already know the spine dimension, I personally prefer to set up a non-facing document withthe page size set to the trim height of the cover and the combined width of the front , back and spine, and add a bleed allowance. I then set two columns with 0 margins and set the gutter between columns equal to the spine. This divides the page into back cover, spine, and front cover areas with the guides.
This is exactly what I do as well. Unless your front and back covers are being printed separately for some reason, covers are usually printed as one sheet and setting up your document the same makes designing and printing much easier. The only annoyance is having to manually add safe-margin guides next to the spine columns, and if you need spine fold marks you'll have to add those manually as well outside the bleed.