I lucked into a dozen of kegs myself recently and am doing the same thing. The first three or four were fun, but now it's just manual labor.
I use a keg tap to depressurize before I cut, and I've made a jig for my grinder to try to make a nice clean circle.
It helped me to know that you don't need the Greenlee Knock-out punch sets that some people use. My cheapo harbor freight step bit has been making nice clean holes for me at a fraction of the cost.