Answer:
The only reason to give a rabbit a bath is if it's messy and can't clean itself. Healthy rabbits groom themselves. Rabbits do not normally need baths. If your rabbit is messy, that means something is wrong! Baths can be dangerous for rabbits, so if you need to give a rabbit a bath you must be very careful and follow certain precautions. See the related question below for details and links.
Reasons to give a rabbit a bath include:
The rabbit is stained with urine or feces. In this case, the rabbit is sick, or perhaps her cage/habitat is filthy and too small -- either way, the rabbit needs to see a vet.
The rabbit got into something messy, like mud, or you spilled something on her, like cooking oil. In this case, you can skip the vet visit although you must pay very close attention to her over the next week or so, and if you notice signs of illness, go to the vet after all. Also, make changes to ensure the rabbit never gets into that mess again.