DO you mean bagging the rear for towing? That is the only reason I can think of someone wanting to put bags in the back only. Will you be doing the front later? If so, I would just save up until I could do the whole thing.
It seems that you are talking you dont know if bags are a safe, reliable suspension. I drive my bagged/body dropped dak daily with no problems. HAve been driving it daily while being bagged for 2 years without problems. You can bag just the back, but unless its only for towing, id wait and bag the whole truck at once.