If you just wait for the dream chances are it will be in your bed basically a false awaking and can sometimes mislead you to think your wild failed, if you try to imagine a dream scene, it can lead to interesting out of this world imagery, but of course you could just as easily drift into non lucid dreaming oblivion. All and all it's easier to let a dream form on it's own, but the results from visualizing a dream during that point can be more rewarding in visual's and scope.