depends on what you're doing :p
there's two schools of thought: big springs, light bars or light springs and big bars
the idea is that you're *generally* using the bar as a tuning aid because it's more easier, inexpensive and less time consuming that changing springs.
in the end, it's all a system and the system has to work together. so having big *** t-bars in the front and light /6 springs in the back makes for an ill handling package because they're not complimenting each other.
a lot of factors go into it that are frequently overlooked: shocks and tires play a very important role as well.