I had a Mesa that had both a three prong cord and the ground switch. there were a couple of times when I would use it to quiet a little bit of hum, but for the most part it didn't seem to make a difference.
When it comes to playing in places I don't know, I bring one of these:
http://www.idealindustries.com/produ..._e-z_check.jsp
I feel much happier when I know for sure... and for the ten bucks one of these costs, it just seems like a mildly smart thing to carry in my gig bag.