This is what my friend Cindi uses on her longarm. Not only does it help her make sure the quilt is centered, it also helps her ensure it stays straight as she rolls it. I gotta get one!
http://www.sewthankful.com/LongarmCenteringTape.html
See how she has the pink markers on the tape lined up with the corners of the blocks:
[ATTACH=CONFIG]293528[/ATTACH]