Mine are rounded - RV4 (elevator only, no horns on the rudder). What are you flying? Of course mine are pointed directly into the slipstream. Other RV?s (7, 8, 9) as far as I know are not pointed into the slipstream, but are nested behind a streamlined leading edge of the control surface. Unless it?s deflected into the slipstream (usually a non cruise trim condition, generally) it shouldn?t matter. If you decided to make that horn streamlined when deflected from normal cruise condition, you are only reducing drag during a non-cruise flight phase. This would effectively make the horn area smaller, and in the case of the elevator would limit the necessary room for the lead counterweight. You would have to add more weight further aft in the horn area, which would require more weight than would be necessary if placed in the nose of the horn, as designed. I don?t think there would be anything gained by rounding the leading edge of the horn, but if you try it, let us know. Who knows, if your horn is constantly deflected so it?s exposed to the slipstream, it might make a difference, but I think the lowest drag cure would be to alter rigging to eliminate this deflection during normal cruise flight.
Just my opinion.