The seam in the center isn't about a seam coming apart. It is that most people start folding quilts in half ... half of a whole is the middle <G>.
Also ... when people use a quilt, they usually put the middle over themselves. Think of where all bedsheets, etc wear out ... in the middle.
It could be that "instructors" are being too over cautious, but the other item is also the line going straight.
A more relaxed mental attitude probably wouldn't hurt in these matters.
A way to allieviate the concern over a straight seam, or that the seam would be really noticeable, is to select fabric that has a lot of pattern to it, as in calico ... the busyness of the little figures hides the seam - - usually <g> ... there are always exceptions to every rule.