I'd keep it vertical. I am pretty sure the paintball tanks have liquid CO2 in them and liquid CO2 would enter the regulator if horizontal.
Primary: Apfelwein, Raspberry Lambic, Experimental Wheat cider, IPA
Keg 1: Hefe
Keg 2: Irish Stout
Keg 3: Peach Kolsch
Keg 4: Empty
Bottled: Bourbon Vanilla Porter, Chocolate Stout