Again, @Jon T
Some of us are speaking from EXPERIENCE. I actually wash my car. Yes, you will get some drips down the door, but that's a directed hose. Water will get UNDER even if you're careful.
I'm sure rain will get in IF it's a downpour and windy. But I have had the car overnight at Carlisle(and we all know it rains there in the spring!) and the interior is dry in the morning. Even with steady rain.
Rain would have to get driven in at a pretty good angle to get under the tonneau in the dash area. There it's pretty protected by the angled windshield. In NY, we have paper registration stickers. My stickers would have fallen off the car or been destroyed if things are as Jon predicts. But in fact, after 17 years, my stickers have all remained intact. That is despite all the water they've seen over time.
If parked overnight I'd put the top up if I had one and the chance of rain was high, yes.
What I did? Put the tonneau up. If major rain predicted, I'd open the hood and clamshell and put a heavy plastic dropcloth over the passenger compartment. I close the frunk and clamshell over the plastic to hold it in place. Works a treat.
Also, I've only had black canvas. It looks great even after all these years. The first one was from 2002 and was still looking great in 2016 when the car met its end.