I prefer the tarp on the diagonal generally, since it gives better protection from the rain and is less claustrophobic - and only needs one pole to lift a side up. Square on is useful if wind is the issue, or tree spacing forces it.
What I do is to thread my ridgeline through the centre loop on the tarp only (and leave it like that packed up). I have two prussiks on my ridgeline with attached mini carabiners, which I then hook to the appropriate end loops, either corners or halfway down a side. That way you can reconfigure it on the fly, so to speak.
You can see it here (DD tarp) - the central loop is directly above my lads head. I had a mini crab on it there, so I could pack the ridgeline separately, but it lets the middle of the tarp drop a bit.