I have never used center sheeting.
My 18sq. had traveler(rear) sheeting.
I was only referring to the crossbar/mainsheet issue.
My 18sq. had straight tiller arms, not the angled arms, so I did lose a few inches of outward travel, but when you are already 11 feet wide it makes little difference.
I am currently using "top" sheeting on my Hobie 17. The cleat is at the boom.
When I switched to a very light mainsheet and 40mm blocks, I found that the line didn't have enough weight to "whip" out of the cleat when sheeted hard.
When trapping low on the wing, it was even harder.
I would have to whip it so many times to get it to release that I would have a very painful "tennis elbow" at the end of a regatta.
I never lose the mainsheet in that pile of line on the tramp either because it is hanging right in my face when I cross the boat.