You can run different combinations of bars. I usually run a 35mm in the front, but I've run both 19's and 21's, even sometimes a 22. There is no one answer to the question, it really is situation dependent, like so many other things in life are.

I'll start up front: I prefer the 35 in most cases, the exception being that the car already has high rate springs (over 550-600), then we stick with a 32. Bars are much more effective tools for controlling body roll than are springs. On another site, there is a thread with pics and you can compare to LT1's, on with 1000 lbs springs and a 32, the other with 500's and a 35mm bar. Both lean the same amount. I like a softer spring for the benefits you get in mechanical grip, ride quality, the fact you don't have to run crazy valved shocks and the like. I make the car flatter wtih the bar.

Other things that you get with the 35mm front bar: Quicker turn in response. Better traction off of tight corners (keeps the inside rear tire loaded better), and the biggest one is that you control the camber curve much better. I run a 35/19 on my own car. I'm not allowed by rules in the class I run to change the rear or the springs. My car does not understeer anymore than it did before. Actually, it's less prone to understeer than it was with a stock bar, about the same as a 32. Again, it's the better camber curve.

When I run heavier front springs, I generally go to a 21 rear bar. UNLESS the car has all poly rear control arm bushings or rear springs over 150 or so. The combination of those an the bar causes more oversteer in my opinion, and definitely hurts corner exit power-down.