I personally believe that a good master word clock source is a nice addition to any DAW setup and can definitely help in critical situations. If you don't have any other digital gear other than your pre's (presonus right?) you can probably get away without one.

The next time you record/mix/whatever I suggest, if you can, is to rent (or borrow a friends, or audition one from your local dealer, steal, whatever) a word sync generator to do an a/b test. If you hear an improvement, great. If not, don't bother buying one.

maybe I will

Thanks John. I will borrow one this weekend. But it's importance is in the recording stage and not the mixing stage correct ? I am mixing 100 % in the box.

It is certainly critical at the a/d converter and less critical at the d/a converter since one argument is that you can have better equipment at the d/a.