There generally isn't a hard and fast rule, and many owners do NOT give baths except in extreme cases. Some owners who have allergies bathe monthly.
There IS damage that could be done to the fur/skin if you bathe too much. The oils are necessary to keep the fur and skin healthy.
As far as smelling, they shouldn't, if they really shouldn't smell if their cages are kept clean, and they are on a healthy diet.
As for buck-grease, a bath doesn't cure it! If you have boys, given them a piece of bread soaked in olive oil once a week to cut down on the buck grease. If buck grease bothers you, you might want to stick with girls. Bathing a male rat solely when it gets buck grease would be too much bathing.