Unless you are hauling, in the bed, something that tends to get one side a lot heavier than the other, you will find better to plumb together.
Also, seems like most want to air the bags up so ride returns to original ht. I found that results in a very rough ride. All the additional wt is then carried by the bags. I have gotten to where I air them up so that the pickup returns about 1/2 way to MT height. That way, the additional wt carried is shared by the oem springs and the bags. Works for me. Better ride too.