Guinea pigs should have a bath now and again to keep them clean and smelling great. Bathing also helps to reduce fungal and bacterial skin ailments and keeps their fur shiny and healthy. You can also use bathing to neutralise personal guinea pig smells when creating new pairs or groups.
How often should I bathe them?
I would personally not bathe a guinea pig more than once/twice a month at the very most (unless medical treatment is being carried out using medicated products), as over-bathing can strip the skin of its natural oils and contribute to dry and fungal skin problems.