I don't use the shoulder straps because they would defeat my purpose. On the MountainSmith Daylight, there is a Delta Suspension System with load levelers. Basically, these additional straps can pull the top of the lumbar pack tight against my back.
I have found with carrying 10-11 pounds in a lumbar pack, the belt and load leveler straps slip over time so once I get them set real tight where I want them (extremely tight), I tie them off. It isn't according to design but neither is my use. I think I could use safety pins or something like that to keep the straps from loosening.