I used to keep it in my left rear pocket.
About ten years ago I changed it to my left front pocket---I find it easier to protect / check on it with my left hand when it's there. That was about when I started carrying more than a few dollars at a time with me.
When I do carry one it is in my front right pants pocket. When traveling I rarely carry one and just have a small id type folder with a credit card, 20 bucks and driver's id.