I've also found that it's a more smooth bend when you remove the resistor. Bending to + and - tends to make things uneven, or give the rotation "spots," when you're actually turning the knob.
What I did on my last one was leave ONE side of the resistor attached to the board (the side nearest the brain), and wired the other side of it to the middle pin on a pot. That way the pitch will always return to its original value, rather than slightly detuned (which is what often happens if you try to go to + & -). But that was on that board, you're just going to have to experiment.
Another thing I've done is put in a DPDT toggle to switch between the two circuits. I took out the resistor and soldered it to two pins on one side of the switch, then attached the two ends from the circuit board to the middle, then attached the other two pins to my pitch knobs. It works great and you can always return to the exact way it was out of the box.
I find it curious that some machines just won't bend up (as far as I know anyway). I've been working on a My Music Center keyboard and every time you try to go up, it crashes. It'll take the pitches down into the depths of hell, but even a couple semitones up and you're done.