Will Audyssey be able to make up for odd surround placement?
Audyssey can adjust for a difference in distance. If one speaker is 6ft away from you, and the other is 12ft away, Audyssey can delay the signal to the 6ft so the speakers sound the same distance from you.
Audyssey can also adjust the volume of each speaker, so that they sound the same level at your seat.
But Audyssey cannot change the direction of the sound. If you put one speaker down and one up, it will sound like one down and one up. I don't think you'll ever see this recommended.
That being said, you need to think about what you want. Remember, most surround sounds are effects... crowd roars, doors slamming, floors creaking, doorbells, car horns, etc. Say you put one speaker high and one low. Sound effects from the low side will sound like they're from ear level. From the high side, the effects will sound higher. Is that a big deal to you? Will you notice? Will anybody else notice?
Frankly, for most movies & TV, I don't think it will make a huuuge difference. Some, not huge. (I am assuming you listen to music in stereo and never use these speakers for music.)
So here's my opinion. If home theater is a secondary concern and aesthetics are more important to you, go ahead and put one surround in the ceiling. If you care more about audio, keep your surrounds roughly the same height. Most of us have violated feng shui in favor of improved audio.