I don't wilt-pruf anymore because I figure if a shrub can't make it through the winter on it own, it's not suited to this environment. So I can't help you with timing.
However, now you've got me thinking - I fall-planted a boxwood hedge and some Japanese hollies and find that with the lack of rainfall this fall, I have to keep up with the watering. Maybe I should apply wilt-pruf in this instance. Are you dealing with new or well-established shrubs?