My right shoulder is generally lower than my left one, however this is because I used a messenger bag for a long time and so to compensate I stood leaning a little to that side. I did it for long enough that I naturally stand like that to some degree. If it weren't for that my shoulders would be even.
My doctor said I had one shoulder that was a little higher than the other. You really can't tell but if you look at it real close it is. But my doctor also said I will grow out of it and it will straighten out.
I've got mild scoliosis, so my right shoulder is slightly higher than my left when standing normally. My doctor noticed it when I was 14, sent me to a specialist, but thankfully they calculated it was only mild, so I would never need to have it treated. No real effect except at the range or martial arts--I shoot right handed fine, and my right side fighting stance is great, but left side in either field is harder.