Both "could care less" and "could not care less" work well in conveying the semantics "I care little".
One says, "although I care very little, there is some wiggle room to care even less". The other says, "I care so little, I couldn't care less than I do now".
Either way, I care little is the main message.
However, "couldn't care less" is more sensible, because what is the point of expressing that you care little, but still have room to care less? That sort of expression would only serve as a retort against an accusation that you do not care. ("A: You don't care at all! B: That is not true, I could care less.") B admits that he or she cares little, but objects to being characterized as entirely uncaring.
We should choose the expression based on its logical sensibility, rather than regional dialect.
I don't know about "could care less" conveying the semantics - I genuinely paused when I first read it to work out which meaning it had.
Let's saying "caring" goes 0 to 10. "Could care less" includes everything from 1 to 10, assuming integer granularity - it's very much the right hand side of the scale, anyway.
Only "couldn't care less" covers that 0 rating.
So - perhaps it's terrible in conveying semantics and only context/tone imply the disinterest being communicated?
