I think that one of the reasons some Estate Planning attorneys keep the original documents is because it insures that they would get future business from the client's family when the Trust was executed. I recommend that my clients keep their Living Trust; Wills and other Estate Planning documents in an easily accessible place. Moreover, I suggest that they inform their successor trustee/executor where they are kept.
Ultimately, I am a believer in keeping the documents at home in a safe. In any event, the most important take away is to inform the successor trustee/executor where the documents are kept.