mmm Yes - glad I'm not the only one using this odd method. LOL Of course, no need to "rub the kitten's/puppy's nose in it" - they will notice when they next visit and it does help them associate the thing with the correct place. Job done - pardon the pun again - LOL
By the way - I have five cats but not six litter trays. Mine seem to cope well enough with four trays - two downstairs and two upstairs.
PS: I do add more when kittens are born though... just depends on how much space you have.