Whilst I completely concur with putting sodium bicarbonate in the bath water IF giving a bath, I would also agree that a nightly bath is not necessary.
I used to say to parents that a nightly bath was more about routine, a little exercise, a pleasant play, and settling to sleep, rather than cleanliness. Some think that it contributes to nappy rash - I personally don't, as long as it's only plain water (sometimes 'bath products' produce a rash).
There are so many causes of nappy rash, you really need someone like a Health Visitor to look at it (even remotely you can get a good idea). Using the wrong one can be useless or even worsen the rash, depending on the cause.