The borders are not necessary so if you prefer not having them I would leave them off. =) The following are two reasons I still include them.
1) several years ago some printers had problems printing edge-to-edge. Depending on paper size you might find part of the cover's short sides cut off by the printer limitations. That is not really a problem with modern printers. The borders gave some "fudge factor" room on the short length. Some folks might still use older printers so . . .
It was also a problem on the long length for some printers. My old HP printer was limited to a minimum of .560" on the bottom print margin IIRC. No one ever complained to me about the long length??? On a 8-1/2 x 11 paper size I needed a .150" top / bottom margin so I couldn't print covers on my HP. The same could occur for other paper sizes on the short length. My completely outdated printing instructions show the margins needed for the standard cover stock that I used back then. http://www.gog.com/forum/general/unofficial_dvd_covers_for_gog_com_games/post733
2) World wide cover stock production sizes also varied greatly and you could find some of the cover stocks smaller/larger. This resulted in the need to cut off (mostly top/bottom) part of the correctly sized cover image. To fix the problem I would have to resize / reprint the cover image again which was more time and ink. Adding a small border at the bottom/top allowed for cover stock variation and also for some non-cover image to be cut off for smaller stock which avoided the reprinting. If the cover image fit perfectly with your cover stock the borders are not really noticeable and on larger cover stock the grey blended well with the existing cover stock in most cases.
Even with the borders removed, if your printout is too large . . then your back to resizing / reprinting. I no longer print covers but I like to save them with my game install files. I leave the top/bottom borders on my covers designs for those who still print them.
On the width of the border I believe it was 25 top and bottom but I haven't checked it in years. I just plopped your image into my cover template and sized it badly hence the difference between the top/ bottom borders on the image I posted for example. Sorry. =(
You cover work is excellent BTW, hope to see more of them in the future.