Makes no difference. As long as the capsule is pointing toward the sound source you should not hear any sort of difference in sound quality no matter how the mic is oriented.

Someone once told me that they heard a difference when a tube condensor mic was pointing the ceiling (with the tube directly underneath the capsule. Something to the effect that the heat from the tube was affecting the capsule). I, frankly, have not heard a difference.

Hi,
Depending upon the quality of the voice sometimes it does make a slight difference.

Males in particular, who have a rather nasal tone could benefit from mic in an upside-down position with the capsule angled forward above nose-level.
However, having the mic in an upright position with the capsule slightly above and angled forward above the nose would offer the same advantage.
There are nevertheless, two practical advantages of using the mic in the inverted setting. The first is, clearing the line of view for the lyrics on a stand and other paraphernalia, and secondly, making the singer look up and give a slightly chestier performance.

It is known that a higher mic placement can add some degree of room ambience too!! (could be an advantage as well as a disadvantage).

On the lighter side, well I guess looking up to a U87 would be more gratifying than looking down upon it !!!

It all depends

The point about forcing the singer to stand up is interesting - but I stopped long ago trying to force the posture of the singer. However they sing most comfortably, that's how they should do it.

Pointing the mic up or down gets more floor of ceiling and that makes a difference depending on how you want it to sound. Getting more nose or chest is definitely a legit placement issue - just do what sounds right. I have a lot of trouble getting nasal sounds out of a mic - usually I will select a less mid rangy mic for a nasal singer.

Position relative to axis can be useful in getting sibilance or mouth noise out of the equation. Will ribbon mics, you can actually reduce noise depending on the orientation of the mic because they act like single coil pickups. hope that helps. ted.