Clearly both are accepted, but I prefer the original.
The main problem, for me, is that I think that those using it want to use the original couldn't, but are just using it wrong. They aren't going for sarcasm, they're describing the level of their care. That's my opinion and basis for why I prefer the original.
http://dictionary.reference.com/help/faq/language/g09.html
http://blog.dictionary.com/could-care-less/