I believe that some folks use heavier lines mainly to straight out snagged hooks instead of loosing a jig. That usually means they are fishing in brush or timber. For most open water crappie fishing, 4# test is probably sufficient. I prefer to go the 2# route, but I'm not fishing snaggy water conditions.

And if you are concerned about visibility tie on a 3' fluorocarbon leader. Gives you the best of both worlds. Myself, I don't necessarily buy into the thought that crappie are line shy, but I still use a fluoro leader as an insurance/confidence edge.