Hey Steve,
I used to run one of those rear rims... it only had one light grind on it though. It was plenty strong enough, as long I was reasonably smooth, and used decent tyre pressures.
The only way you're gonna know when the sidewalls are too thin, is when it dents easily!
So, you can either just keep grinding away, or keep it smooth and put up with slightly inconsistent brakes.
Hope that helps,
Adam