Chris, in Rhino when you trim on straight lines its faster if you use the "cutting plane" function (surfaces).
If you work on a whole hull the volume centroids would be on the centerplane if you work with just one side of the hull the points would be a little on the side but still in the right position, eventually takes nothing to project the points on the centerline, or even physically move them using the snap functions.
If you position the model right and use the measure units youll work in, the coordinates of the points will tell you all the measurements you need right away without having to use the measuring tools the software has.