Hello,
Like Sheldon said, if the side straps are adjusted properly, it will stay perfectly centered. Here is a photo of mine while raising the mast on my Prindle 18.

Mine works out that I can push the adjuster loop of my trap line through the fairlead that the barberhauler block is fastened. The hook on the E-Z-Step strap clips on this loop. The mast and pole will stay in exact allignment every time.
Good luck,
Jack Hoying
Fort Loramie, Ohio
Prindle 18 #1645
http://www.bright.net/~jmhoying/sailing/prindle.htm