I agree with stxranchman and start rattling 10 days to two weeks in advance of when you think the rut will begin. The bucks will have much more vigor at this time and therefore will be more responsive to rattling. If the buck to doe ratio is very low, you will most likely have a rut that runs over several weeks and is pretty hard on the buck herd as they attempt to breed the does. You will also see many immature bucks breeding does that would not happen if the buck to doe ratio was closer.
If you get a chance, pick up a copy of the July-August edition of the JOURNAL OF THE TEXAS TROPHY HUNTERS magazine. I wrote an article about common sense rattling in the Hill Country but the same principals will apply across the country.