I've never done it, myself. The way I think of it is two-fold:
1. I don't shave off anything other than my finisher, so for me test shaving off anything less than my finisher (for example to compare sub-finisher stones) is pointless;
2. Whenever I shave I always strop. To me it is how a finishing stone behaves in conjunction with a strop that matters. If I want to compare the edges of finishers, they get compared through the lens of a strop: to me, a shaving edge is a stropped edge.
I'm not saying I'm right, I'm just saying that's how I think of it. I guess it boils down to what you think a strop does. I will say, though, that if you think a stropped edge negates or removes the differences in edge-feel between two finishers, then you've got to wonder why we worry about the relative merits of finishing stones at all....
James.