Just another thought...
Yes the seam in the middle might get a bit of stress... but...
You are quilting along this seam. You are quilting across this seam. Every point where you have thread tieing the front to the back reduces the stress along that single thread. Do we have any emphirical evidence of seam separation along a center seam quilt back that has been adequately quilted?
Inquiring newby wants to know (as he goes out to get some 54 inch backing fabric).
tim in san jose