No rear bar, yet. However, this is because I always intended to also use the car on track, which I now have. Too many have already been down that road and found the rear bars can make the rear loose at higher speeds; Steve (autoxcuda) most recently... Talking A-body only here.

But it all depends on the total setup. Andy mentioned Werner's red Valiant, Mapes has one on the Zebra Dart - with a panhard. So it all depends on the roll rates, CG, weight distribution, etc.

I think with a soft enough rear spring rate (relative to front) a light rear bar may be helpful in autocross. 5/8 or 3/4" with adjustable positions on the arm because a little is a lot.

If you want to get technical about it, you can play with various rates on the neutral line for your car.
It's something I'm thinking about trying later this season, as I need to replace the rear springs anyway.