Based on the asteroid maps that have the two lobes on either side of the big asteroid, I'm assuming it's not required. It would look odd though unless you had a binary pair or something.

At least in Sins 1, ships could accelerate down a well faster than out of it. I've not checked Sins 2, but I imagine that if that is still the case, it'd complicate that math a bit.

Lore wise, the phase lanes are blocked by gravity wells, so it makes sense they are in center, however some asteroid clusters have their mass more distributed creating other shapes (see below). There may be other variants in the future. 

So much could be done in a Sins w/o phase lanes now that planets can orbit themselves.

Lots of RPG possibilities. Select a crew on a single ship or strike group and explore and battle....the only phase lanes would be to a different star where every star is a harder "level".