I'm no engineer, but there's no way anybody could convince me that an out of round inside the lugs, at the wheels center could possibly effect balancing at the measurement levels of a tire shop. the rotational mass at the center is so small, that it wouldn't be a difference.
By using a mounted point in a grinder, you are spinning a stone that is nearly the same size as the hole you're looking to achieve. Not like using a small bit of a die grinder or something. The biggest concern would be making sure that the cut is clean and smooth, without chips and chatters, that could lead to radial cracks.
I looked long and hard into this recently. I didn't wind up with the wheels that needed it, but I wouldn't hesitate to transport my kids after I'd done it. <img src="/forums/images/graemlins/kewl.gif" alt="" />