If the fur is not soiled at all, have you checked the scent glands to make sure that isn't what you are smelling? The scent glands can get a build up of a waxy substance that sometimes needs to be cleaned out. And the smell can be quite overpowering. Also an unspayed/unneutered rabbit may naturally have a stronger smell due to hormones and an increased need to scent mark.
http://brighteyessanctuary.org/educ/rabbit-info-scent-glands.html
If the fur is not soiled, there is absolutely no reason to be giving your rabbit a butt bath. Bathing a rabbit at all is generally not recommended and
a butt bath should only be done when it is an absolute necessity due to a very soiled bottom that can't be cleaned up by other methods. And if a rabbit is getting a soiled bottom, then a cause needs to be determined as well, as a healthy rabbit should not have a soiled bottom at all. When a rabbit is kept in clean conditions, a dirty bottom is an indication of a dietary or health related problem, and the rabbit likely needs to be seen by a good rabbit vet.
Bathing of any sort can be very traumatic for some rabbits, even to the point of being fatal. There is also the risk of the rabbit either overheating or becoming hypothermic if the bathing procedure is not done correctly, not to mention the risk of fly strike in warm weather when the rabbits fur is not dried completely. I would not recommend even a butt bath, especially in the wintertime, unless the rabbits bottom was extremely soiled and there was no other way to get it clean, in which case the rabbit also needs to be seen by a vet right away. If it is only lightly soiled, it is much better to use unscented baby wipes or a damp rag to try to clean the soiled fur. Afterwards the fur needs to be completely dried before returning the rabbit to it's enclosure or outdoors.
If a butt bath is necessary due to the fur being very soiled, extreme caution needs to be used and ensuring it is done correctly. It is also very important to make sure the rabbit does not get too cold or hot, keeping a close eye on the rabbits body temp, making sure the rabbit is warm enough until the fur is dried completely. Especially do not put a rabbit back outdoors in cold weather, with even slightly damp fur. The fur needs to be fully dried.
And it is best before any bathing is attempted, that the rabbit should immediately be seen by a vet to first address the reason for the dirty bottom. If this cannot be done immediately, the rabbit may need to be temporarily cleaned up and precautions taken for fly strike, if in warm weather.