The problem with looking down a barrel from the muzzle end (btw, I can't tell from the post if the OP looked down the barrel from the muzzle or breech end), whether on the range or in your home, is that it teaches or programs you to do something very dangerous. Or it will teach someone, who may be looking to you for guidance, such as your friend/child/student, that looking down a barrel is ok.
Even if I knew that a gun was not chambered and open, I would never do it. As they taught me in tactical school, the way you learn and train is the way you will do things. If you get into the habit of looking down a barrel, one day you may be this guy:
► 0:52► 0:52
www.youtube.com/watch?v=RGlbibgmg2o
Oct 30, 2012 - Uploaded by quicklolcom
Before you say, 'oh that guy's stupid, that'll never be me', you shouldn't be so sure. Human beings often do things half consciously and on autopilot all the time. For example, almost everyone has driven home from work, and then, after getting home, not even remember driving home. If you shoot often enough, you'll eventually just act out of habit without thinking.
If you must check a barrel, open the gun and look down the breech end.