If you're using ABS then it is largely unimportant, but you should not set it too low if you use ABS as it will simply reduce your braking power, when it really makes a difference is when you are not using ABS, in which case to avoid locking you should have it fairly low (3/1 or 3/0 on rear wheel drive cars, for example).
The higher the pressure, the easier it will be to lock the brakes up, if the rears are set too high then the rear tyres will lock up easier, causing oversteer like you just pulled the handbrake, where as if the fronts lock up or are too high (harder to lock the fronts, so higher pressure can be used) you will understeer and slide.
Generally, higher brake pressure on the rear can help you turn in, useful in understeering FF cars, but on FR/MR cars you really want the rear bias to be lower. But keeping in mind, with ABS on you cannot lock the brakes at all, but bias also has a much much smaller effect, if you use the same low settings (3/1 for example) as you would use without ABS you will feel like you simply lack braking power, where as without ABS the setting would be perfect for many cars.
Most of my FR/MR cars are 3/0 or 3/1 without ABS, 5/5 with ABS. For the Bugatti Veyron I found that 10/10 (with ABS) was very effective. So experiment.