I always thought "crabgrass preventer" acted to prevent the crabgrass seed from germinating in the spring. Applying it now wouldn't do a whole lot for you. With warm weather, yes there will probably be some of it trying to germinate, but crabgrass will die off when it gets cold.
I know some of the chemicals are not so discriminating. If you are trying to overseed your lawn, they will also prevent the "desirable" grass from germinating as well.
Best to consult your local garden center (not the big boxes, but local nursery) to get their recomendations.
Also I've heard that crabgrass will not germinate if it is shaded. So if you get regular grass going (probably sod at this point, too late for seed to get established) next spring the nasty crabgrass seeds will be shaded and prevented from sprouting. Of course that means they will be there waiting the next year, and the next.....