Will, two practical aspects I can see with an off set snap line, say 2 inches off the reference face, is that a line has been set for one side of the mortice and second it helps(at least it helps me) to keep the ref faces orientated easily, at a quick glance I can tell the orientation. When dealing with center lines you have to rely on some other indicator. Just keeping it simple, it matters not as the function is the same where ever the line is placed.