Im sure someone will come up with a better answer, but this is what the tackle shop guy told me.
You use mono leader on braid lines because the braid doesnt have any slack at all and if you get a bite and you strike too hard, it might rip the hook out of the fish' mouth.
I put about 6ft for jigging, and 3-4ft for shallow water.
Someone please correct me if Im wrong and Id also like to know what others do.
GO EELS