I start having flavor and vapor diminshing after 6 or 7 refills, so I do clean them.
First, make sure you remove all the mouthpieces, and set them aside. You may want to rinse them off, or soak them in alcohol.
I soak the cartos in a pure grain alcohol (PGA) bath for a few hours - you will be AMAZED how much crap comes out of them.
Then I rinse them in water, and boil them for about half an hour (make sure you don't let the pan run dry)
I blow them out, and set them on my window sill with the window fan going to dry for a day or two.
I recommend you use PGA, and not rubbing alcohol, because the filler in cartos can't really be removed and replaced, so in a confined area, you can never be 100% sure they're dry.
vaping a little bit of PGA is fine - but vaping rubbing alcohol is NOT fine.
However, using rubbing alcohol for mouth pieces, atomizers, cartridge filler (removed from cart), cleaning battery threads, and empty carts is fine. Allow to dry overnight with good air circulation, and the alcohol will evaporate completely and safely.