I have 3 boys, and each one of them preferred my left breast to my right. The let down on the right was not as fast, and they quickly came to figure that out, crying & fussing when I'd put them on that side. With my first, after about 4 months, I ended up nursing him solely on the left side, for about 6 months. My Dr. said it was fine as long as he was thriving, which he was. When I stopped nursing him though (at 10 mos.), my breast were uneven in appearance. It was only noticible without clothes on, but it bugged me. So, with each successive child, I nursed them longer on the right breast - trying to always start on that side. They would still each reject it eventually, but later & later. I nursed each on until 10mos., and my second didn't reject it until about 6 mos., and my 3rd didn't reject it until about 8 mos. It has now been almost 2 years since I nursed, and I don't feel there is a noticible difference in the sizes of each breast in comparison to each other (in comparison to how they USED to look pre nursing is a whole different issue!!). I have to say, I do think it's weird though - I am also a type I diabetic, and when I prick myself to test my blood sugar, for some reason I get more blood on my left side too! Sometimes I can't even get a drop out of my right side! Finally, it didn't help that I am right handed, so having my right hand free to hold a book or phone or drink was appealing when nursing.
Hope that helps!