If you are using a 5, 6, 8ft SMB you really need it. The drag on the line and SMB itself can be substantial and you might actually need an extra 2lbs...
Going a bit on a tangent here, but would you typically carry and deploy from depth a larger SMB such as an 8ft? I guess you'd want the chase boat to be able to see the bag in fog/chop, but do people typically do this, or do they shoot a smaller, more manageable bag on deco and keep a large marker for surface deployment if needed?