I bath them almost every night, with the exception of saturday night (don't know why - just mine and DH's way of seeming like we are having a night off from the grind I guess!), and occasionally we don't bother if we are back late, have visitors, or if DD is beyond tired. We wash DD and DS's hair once a week - mainly because it is such a debacle with DDs long locks and feisty temperament!!!
I know some people who are religious about bathing the DCs every night without fail, and others who are a bit more chilled out. However I can't help feeling a bit bad when I decide not to bother for a night!!!!