Well here is a related topic.
I windtrain on weekdays. And I ride on the road during weekends.
I used to clean my drivetrain on Fridays. But now I do it after the 2 weekend rides and on Fridays (after the windtraining sessions).
The chain is far dirtier after the 2 road rides. I has very little dirt on it after the windtraining session which come out to almost twice the hours. So must of the dirt comes from stuff the bike picks up off of the road. I suppose if I were to clean and lube once per week, I should do it on Sunday to clean off the road grit.
I suppose there are 2 reasons for cleaning the chain after a rain.
1) you will have potentially a lot more dirt.
2) you might have washed away the lube.