It's not supposed to matter that much. They work by mounting perpendicular to the muzzle; it's the angle (should be exactly 90 degrees) that's important.
Try this, if your barrel is short enough to reach - mount the collimator as intended, and look through the scope as you move the collimator around a bit. You'll be able to see when it's tilted, but up/down and side to side movements shouldn't have very much change in what you see through the scope.
With that said, I've gone to just bore sighting (visibly looking down the barrel) when possible because I get better results. I did have a Leupold collimator that worked well until somebody dropped it; despite no visible damage it never worked well after that. I suspect the magnet got knocked loose in the base and no longer holds true.
