This debate will never stop.
Our rule is no more then 2 -1 " lines in a trench ( Though we use poly and pull our pipe so this isn't much of an issue)
If you have more then 2 - 3 lines in one trench, then your manifold placement is in the wrong spot.
Both Manifolds and single valves have there pro's and con's. As far as the historic argument on this forum. I would say it's probably a 50/50