Isn't that a thing of the past? We have 3-prong plugs these days which takes care of that for us. If you've got some amps that still use the 2-prong power cable, you should really get them converted because it can kill you.
That is correct... old school amps used these to potentially reduce Hum... newer amps with a three prong power chord have a ground reference (the third prong) and so simply do not need a polarity switch... They may however at times need a ground lift if your getting a ground loop between amps etc... this is a simple mod if you wanted to do it but in this situation I would just recommend carrying around one of those simple ground lift adapters you can pick up at any hardware store for about a buck...