Do you know just which cat(s) are doing it? Can you tell if anybody is drinking more water than before?
I have just always heard that any behavior change in a cat should be treated with a trip to the vet. Cats are so darn good at hiding illnesses that the change may be all you get until an illness has progressed too far.
It is probably nothing but play, but it never hurts to check it out.
My motto is; better safe than sorry.