I am sorry, but I disagree with this. Yes, it is how it sounds when said verbally, but we should still be teaching the correct written grammer to our children (or peers who are learning English, or how to read/write etc). I don't think it has been adapted as a correct form of grammer.
^ yeah, that!

Would've and should've just sounds like 'of' when spoken, but grammatically 'of' is incorrect.