In order to get shocked you would have to have a current pass through some part of your body. Typically this happens when you are standing on the ground and then touch a wire that has a voltage respective to ground. In this case you would have a potential difference between your feet and your hand, and current would flow through you.

If you are insulated from the ground, you can touch a live wire and not get shocked, because there is no path for current to flow. Likewise if your feet are at ground potential and the wire you touch is also at ground potential no current would flow because there's no difference in potential. 

Imagine taking a 120 V battery, connecting two wires to the negative terminal, and holding one wire in each hand. Since both wires are connected to the same potential you won't get any shock. Now connect the wires to the positive terminal, the same thing would happen, same potential, no shock. It doesn't matter if the potential is positive or negative with respect to the other terminal, as long as you don't connect yourself to both terminals at the same time there will be no path for current. This is the same for AC, it doesn't matter if the polarity is reversing, as long as there is no difference in potential across your body there is no current.

(Disclaimer: I'm not suggesting you actually try any of this, doing it incorrectly is likely to be fatal. Line voltage is nothing to play around with.)