Yes, either phrase is usually understood by American readers. However, "I could care less" has a literal meaning opposite from its idiomatic meaning and is likely more troublesome for non-native or non-American readers.
Also, when many people read "I could care less", they assume the writer has a deficiency in their education and/or attention to detail. Seldom is it advantageous to write "I could care less" instead of "I couldn't care less".
Also, when many people read "I could care less", they assume the writer has a deficiency in their education and/or attention to detail. Seldom is it advantageous to write "I could care less" instead of "I couldn't care less".