There are no hard and firm rules for the placement of a UPC bar code on a saddle stitched comic. We choose to put it on the back cover if you have a Ka-Blam ad because it’s an inconspicuous spot that doesn’t mar the front cover image and we allowed space for it in the design of the ad.
However on paperback books it’s different. The ISBN guidelines say the bar code must be printed on the BACK cover of a book. It can be printed elsewhere as well. Many books also include a bar code on the inside front cover. But the back cover is the required location. So if you’re prepping a paperback be sure you leave a null space about 2 inches wide by no less than 3/4 of an inch high on your back cover for the ISBN and bar code.