Great query....with....I'll bet...no clear answer.
On my prior tin, it was mounted nearly parallel to the port (left) side. I never noticed any loss of efficiency by it not being dead center.
When I canned that old hull, I paid a local shop to move my trolling motor; big engine; and console over to a new 1652 G3 hull. I ASSUMED that the installer would put the TM in the exact same place on the new hull.
Nope. He put it diagonally across the bow, kind of taking up more space. The power is now dead center. Perhaps there was some gain in efficiency. If so, I never noticed it.
I've gotten so used to it that....this weekend...I bought another larger trolling motor and just installed it (an hour ago!). I used the very same holes that the installer used when he moved my former TM over.
As you can see, I don't think that there is just one perfect answer to your query.
Clearly, at least for me, if you cast with your right hand, you probably want the trolling motor mostly on the left (port) side. That's my story and I am sticking to it.
regards, richg99