A Wah is a negative earth pedal and its v-, not its v+ should not be in contact with the casing. In a positive ground pedal it's the v+ that's connected to the case but the 9v+ ground being a reference point you should not be getting a shock when you touch a positive ground case and your guitar. You are basically touching the same reference point because the voltages of both the positive ground and negative ground pedals are not directly touching your guitars hardware or pedal's enclosures.
I don't have a degree so there are maybe others who understand and can explain it better than me.
That said, I have had mild but annoying electrical shocks when touching my pedals when I ran a Pedalboard with a combination of 9v,12v,18v and 24v negative ground and 9v positive ground pedals on it.
I made very sure everything was in good shape, especially the amps and power-supplies and it was, but I was still getting these mild shocks, that felt a bit like static discharge.