It's one of my favourite series, both as book and tv! Letting Bertie swear would be funny for sure, especially when mixing in all the other slang like "sporting" he uses. I also think it would be useful to use when talking about aunts - Aunt fucking Agatha sums it up pretty well. Other good swearers would be Honoria and Aunt Dahlia.

Jeeves can also swear, as a little treat. Probably only when he is thrown off completely to underscore how serious the situation is. :D