Unless everyone you know speaks grammatically correct and regardless of how often and how annoying Word is about trying to correct you – do not use grammatically correct sentences in your dialogue. Even very educated people speak differently than they write.
And to further the ‘natural feel’ of your book, be consistent. If your character is uneducated he/she will not speak the same way as someone who is educated. A street person will not speak the same as a housewife. And the list goes on and on.
This is a very important to follow when writing your story. It will also help develop who you are trying to portray.