I've heard that they can actually be harmfull in certain situations. Something about loading the weight in the wrong direction and causing a roll-over...
I would just get nice set of greasable shackles, maybe 1"(2"max) longer than stock to add a little extra flex without being unpredictable.
You could also take apart your spring packs and spread a good coat of anti-sieze lubricant between all of the leafs, and inside all of the spring bushings too.
If that isn't enough, you may want to remove a leaf from each pack...