I carried a double closed-top mag pouch horizontally, on the gun side, with the tops facing the belt buckle during the academy and through probation. This is how 90% of my department carries them.
I then switched and now carry the mag pouch vertically on my weak side, just to the left of the gig line. It is a little less comfortable during reloads then horizontal carry but it's not a big deal. The real reason why I switched was to free up some extra inches on my belt. I don't consider a 34" waist to be "skinny" but I'm just out of real estate on my belt. It's also easy to access my mags with both hands.
Incidentally I've also found that the space between the top of the horizontal mag pouch and my uniform is a great place to stuff my gloves or other items in a hurry when I'm done with them but don't have time to put them away.
I know a cop in my county that wears his mag pouch vertically in the small of his back....
I guess he doesn't expect to ever need to reload in a hurry.