Well, it's one reason why I searched for a car without sunroof. One less thing to worry about.
Water will enter inside doors, and inside the rear quarter. All cars with moving windows have this, even modern ones. For that reason there is this plastic sheet in each location with moving windows.
Doors, and our sill also, have drain points for that reason. The rest of the car should be pretty tight.
If your front-seat window seals leak, replace em. Vents windows, heater box seal, likewise.
I think you can wash an e9, but some will fare better then others.
Before starting my restoration i drove it for 4 years and I simply did not care about any moisture. It needed full sills, and thus I did not care if there was 10 pounds of rust, or 10.1. (I even did rally trials, ending up with a few pounds of mud *on* my luggage in the trunk as the holes behind my rear wheels were already substantial)
Now if you have survivor e9, I would take the damp cloth approach, as mentioned above, no question about it. Panel seams such as floor to sill have no rust prevention between them. It relied on the thin paint layer on the panel edges. The paint on these seams cracks often due to body flex, and when water enters those seams, there is no way to dry it out.
If you have a fully dipped shell with 21st century rust prevention, then I would drive it in the rain every day, as long as I could park it inside to dry up.