Steve, you're going to get so many answers on this one that I think you should just pick the answer you like right now and run with it.
In theory a nice pointy hind end on an airfoil makes everything smoother. In reality, I haven't noticed a huge difference. My gut feel is that whatever you gain in better aerodynamics, you'll lose it right back again with a less rigid structure, unless you go hog-wild with CF reinforcement or some such. I know that my Fancherized Twister has flat flaps (I did make a pointy elevator), and I've broken 500 points with it.
I suspect that if you just build the thing per the plan that Sam won't be getting held back by the airplane.
OTOH, if Sam's the one driving the mod -- well, it's her plane. Just point out that she'll be making things more floppy if she tapers the surfaces.
Or make the surfaces removable, and make one set of each, then compare. Samantha can use the data gathered for a winning entry in a science fair, and she can write an article for Stunt News.