on the question of having a seam down the ridge line, i remember the instructions that came w/ my 1st home made tarp (a ray way).
they talked about how much the ridge line (folded, sewn, & sealed) increased the strength of the whole ridge line (thinking from guy line to guy line, including the attachment points for the guy lines). and i believe this to be true.
not only does it seem to help give a neater look, but in a high wind situation, i'd feel much better w/ the extra strength of proper seam in the ridge line than just having the guy lines attached to the edge seam.
don`t leave the CREATOR out of the creation!