The mixed grip is pretty much standard among powerlifters because it helps prevent slipping more than a "regular" grip. Personally I've never liked the mixed grip though. When I hit
heavy weights (315+) I feel as if I'm twisting my spine ever so slightly as I come up with the bar and that can't be good. When I use a regular grip I don't get that so I stick with those. (Some people have also questioned whether a mixed grip might cause different recruitment patterns for the back muscles and different stress for the two sides, but there's no research that I know of that discusses this.) If you don't feel any twisting when using a mixed grip, stay with them.