Not a McGregor, but an ODay Mariner (19ft). Have added goal posts to the rear of my trailer to help the line up when winching her home. However, the stern of a Mariner is a fair amount narrower than the maximum beam. Of course, the max beam is what the posts must be set to. Nevertheless, the max leverage for the goal posts is at the rear of the trailer, AND the max beam tends to be pretty close to the rear when line up is most critical.
The winch should be centering the bow during retrieval, so I'm not sure how effective mid-trailer goal posts would be. I would start with rear pair, and see if you still need mid-trailer pair.
Haven't launched or retrieved with the goal posts yet, but every Mariner owner who has goal posts swears by them. Without the goal posts, I've sworn at not getting correct line up during retrieval.
I know goal posts are essential for my boat lifts, and since there is no winch to center the bow, I have them fore and aft. On my 19ft powerboat, the goal posts also provide fore and aft reference before I lift the boat. Line up the gas tank filler with the forward goal post, and I'm in the right position every time.
Fred W
Stuart (ODay) Mariner #4133  Sweet P