With the hundreds of rats that have gone through my doors, I've never needed to bathe a rat that wasn't very ill. Sometimes, a very sick rat will need help cleaning themselves, but even then I've only ever given them a good scrub down with a baby wipe.
Baths are usually scary and stressful for rats. Even if they seem to like the bath, it really dries out their skin, and can make them itchy and flakey. When it comes to males, bathing actually increases buck grease production, and increases smell.
For rats, grooming is a very social thing. I've read that rats groom themselves and cagemates from nose to tail 10-15 times per day. That's a huge amount of their day. Grooming is used as a way of keeping social hierarchy. Rats are very clean animals on their own, there's no need for us to intervene. I've had rats who marinade (pee and then sleep in their hammocks) and even they clean up within 10 minutes of waking up.