Rats are very clean animals and certainly don't NEED to be bathed.
Though a wetwipe along the tail is often necessary in males who aren't as good at cleaning once they get older.
Keeping their beds and hammocks clean will help to keep THEM clean.
Some people are very anti bathing rats but in reality, some rats do enjoy water. Some don't.
I don't bathe my rats but also don't see the harm in it if they enjoy it.
I'd personally rather just allow my rats to play in a shallow tub, for example 'pea fishing'.
Some don't even enjoy that and I can't really imagine any of them enjoying being immersed in water.
That said, wild rats are well known for swimming and they choose to do so.
As with any animal, if you were to shampoo a rat regularly, you will strip natural oils from the skin and coat. This could result in skin problems.