Yeah, if you have sand sifting cleanup crew, I wouldn't touch the sand bed. If the sand is still dirty then that probably means you just might need to add a few more sand sifters. In any event, the only time you would really want to vaccum the sand bed is if you didn't have a clean-up crew to do the work for you
Edit: Nick (shallowreef) is a weirdo! Do you agree with me Nick???