I have even put a seam up the back of a quilt I entered in a quilt show. It got a first place ribbon, so I guess it isn't such a bad thing all the time. About half the time I have seams up the back, and I don't even care if it is right up the middle. I quilt my own on my domestic, so that probably makes a difference. I know that long arm quilters prefer not to have a center seam up the length of the quilt. (I also SITD, and have won ribbons with those quilts...no real rules in quilting...just serious suggestions, I think.
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If I was making a quilt only to go in a quilt show though, I probably would avoid seams in the back. But my quilts are to use, and the quilt show entries are only really "for Show and Tell." However, if a judge wants to give one of my quilts a ribbon, I will take it...
Dina