It is indeed said that most of the times you want a higher roll centre on the rear than of the front. It is said that on the front it should be about 3/4 of the rear.
An interesting text on a forum I found about roll centres:
The front part of the chassis is forced to hinge on the front RC, and the rear part is forced to hinge on the rear RC. If the chassis is rigid, it will be forced to hinge on the axis that connects both RCs , that axis is called the roll axis ,The position of the roll axis relative to the cars CG tells a lot about the cornering power of the car; it predicts how the car will react when taking a turn. If the roll axis is angled down towards the front,( Rear Roll Centre Higher that the Front RC ) the front will roll deeper into its suspension travel than the rear, giving the car a 'nose down' attitude in the corner. Because the rear roll moment is small relative to the front, the rear won't roll very far; hence the chassis will stay close to ride height. Note that with a car with very little negative suspension travel (droop) the chassis will drop more efficiently when the car leans over. With the nose of the car low and the back up high, a bigger percentage of the cars weight will be supported by the front tires, more tire pressure means more grip, so the car will have a lot of grip in the front, making it oversteer. A roll axis that is angled down towards the rear will promote understeer. Remember that the position of the roll centers is a dynamic condition , so the roll axis can actually tilt when the car goes through bumps or takes a corner, so it's possible for a car to understeer when entering the corner, when chassis roll is less pronounced, and oversteer in the middle of the corner because the front RC has dropped down a lot, In terms of car handling, this means that the end with highest RC has the most grip initially, when turning in, or exiting the corner, and that the end with the lowest RC when the chassis is rolled will have the most grip in the middle of the corner. a very high roll center in the front will make the car turn in very aggressively, but understeer in the middle of the corner. It's nice if you like an aggressive car you can 'throw' into the corners, but I doubt it's the fastest way round the track. Conversely, if the rear roll center is set very high, the car will turn in very gently, and possibly oversteer after that.
I believe that during cornering the front roll centre on a Sierra will drop, due to the angle of the TCA changing.
On the rear I believe the roll centre will go up during cornering. When the body leans in the corner, the outer pivot of the outer arm moves down more than the inner pivot of the outer arm. This means the instant centre moves up so the roll centre also moves up.
So during cornering the rear roll centre might actually still be higher than the front, although stationary it is the other way around.