CoyoteHunter and MM are spot on IMO...there are times when more magnification is a very good thing,especially for deer-sized stuff in open country,and I use a variable topping at 9-10 myself occaisionally...and it's helpful as my eyes get a bit older....but there are very few hunts on which I would not be pretty comfy with a regular old fixed 4X...
I shoot 600 yards frequently with a 6X...
But like everyone says,all eye balls are different!