IMO, a bike should be dirty when it comes back from a ride but never when it goes out! About 70% of my riding is on paved roads but the other 30% is on rail trails that are a mixed bag of gravel/stone dust/dirt. I keep the grit off my drivetrain by using paraffin wax as a chain lubricant so that needs very little attention. The frame/pedals/etc get a quick spritz with WD40 and a rag after most rides. Off-season I'll give the frame a good going-over with carnauba wax.