Majora on 19 May 2012
That graph highlights the phrase perfectly, although it is trying to be witty and smart. The whole point of I couldn't care less is that it really is impossible to care less as you don't care at all.
And yes, I agree, could of, would of, should of = Big no no. Was in a test I did at work recently - amazing how many people don't realise it's could have, would have, should have.
