I have to agree with Casper and Kitty. Unless your cat has gotten into something VERY filthy or smelly, it's best not to bathe them. Cats are very clean animals by nature and usually groom themselves very well.

The only cat in my house that I DO bathe twice a month is my Sphynx, MooShoo. Because he is hairless, the oil on his skin has nowhere to go so it stays on the surface of his skin and causes a ring around his neck. It also causes his ears to get VERY dirty. Those I have to clean out twice a week. Moo has gotten used to baths. I put a small towel in the bathtub so he can sit on it and not feel like he's slipping. Then I just take a wash cloth with kitty shampoo and scrub him down. It takes me all of 10 minutes and bingo.....he's clean and smelling good.

So, unless your cat rolled in poo or is filthy, it's best to leave the grooming to them. Unless, like you said, they can't groom themselves properly. But it is up to the individual.