I understood completely. The OP never stated that "water" was ever held in those jugs. They could be brand new.. which still has residue from some foreign chemicals, or it could have other junk in there. Also, just because it is a water jug, who's to say he could not have been holding dirt.. chemicals.. fertilizer.. weed killer.. and the list goes on. His reason for wanting to bleach it may not be evident, but his reasons should not concern us. What concerns us, is that he knows how to properly bleach the water jugs since clearly that is what he wants to do.
The fact of the matter is.. he NEVER stated what WAS and WAS NOT held in those water jugs.. so it is our job to assume that there WAS foreign material in there previously, this way we can tell him how to clean it out correctly. He asked us if he could clean out the jugs with bleach and rinse them and let them dry. So we either tell him yes, and how to do it, or we give him a better suggestion than using bleach. Simple as that. And from what i can tell.. he has never bleached these water jugs before since he is asking us how to do it in the first place. So that pretty much throws out your last theory.
Also.. let us assume that you are right and they held "water" before. How are any of us supposed to know what was in that water? And how would he know either? The water in those jugs could have been sitting there for a while and accumulated algae, mold, whatever. It also could have acquired other chemicals on purpose or by accident. We just don't know. So with that being said.. even IF before he started using the water jugs he bleached them.. there is STILL all those harmful things in the water jug that have accumulated mold, algae, etc... SO, it would be WISE to bleach the container once again so no residues or any substances that were previously in the jug are left over, which in return would contaminate the new water or substance that he intends to store in the jugs.
Don't just assume they held water.. or he bleached them before.. or he has been using the jugs for a while. It is not the repliers job to ASSUME things. Our only job is to answer the thread OP's questions to the best of our knowledge. And if you don't have that knowledge to answer the questions.. then don't. It would only cause more harm to the OP and others that may be reading this thread, if they followed directions that were wrong or incorrect.