Four Static Ports! Which to Use?

I need you opinion on which static ports to use on the DG. The
friendly folks at Glaser-Dirks provided me with two pairs of static
ports on my DG.

One pair is near my toes, the second pair near my knees. The only
difference is that the aft pair are on a surface that is almost
parallel to the line of the ship. The foreward pair are on a surface
that about +10-15 degrees away from parallel.

So which to use? It would seem to me that the forward pair, by virtue
of the fact that they are not on a parallel surface, might receive
some positive pressure. I assume that positive pressure would be bad
for a static port.

I should rig something up so that I can flip between the two while in
flight to see if there is any difference in instrument performance.
This makes the rash assumption that I would recognise the difference
if I saw it.

BTW: I am using the forward pair.

On a related question, why do static ports come in pairs? I assume it
has something to do with eliminating errors caused during slipping in
which the ports "see" different static pressures.

Thanks, John