I did rebuild a freewheel once - can't remember the reason but given that I was a mechanic and service manager for over 20 years that gives you some idea of how common it is to do so. I would not even bother with cleaning a freewheel without first checking the wear on the chain and looking at the small cog to see if it's worn noticeably. If the chain is worn so that 12 full links (inner/outer) measures more than 12 1/8," or if the small cog is worn there's no sense in working on the freewheel in my opinion - replace both.