you guys are talking about maybe the stairs door being closed if possible sometimes right? I know this isn't always the case(is with our house...actually the top of door can unlatch and swing open too like a horse barn door).
in the bedroom the BG can't see the target(you). you can shoot that fatal funnel when you want(the door entrance and you easily know when the BG is breaking that threshold. the problem with the stairs is the BG's target is in a much smaller place if you are at the top of the stairs. you would also have considerably less time to shoot the BG when SHTF and the BG's target circumference seems much bigger all of a sudden. Now if you are already stationed up there that helps but usually if there is a break-in you would have to get ready after-the-fact.
"Damn the torpedoes, full speed ahead!" -Admiral Farragut @ Battle of Mobile Bay 05AUG1864