difficult issue
I assume that moisture is in the air (humidity) always, so corrosion, if exists, will made it´s way on and on no matter which means you use, unless you have a dehumidifier in your garage
if a part of the car is not rusted the humidity in the air will do no much harm to it, but if it is a rusted area, nothing can be done (another question is the resulting corrosion speed, depending on the humidity %, temperature,...)
of course one on the worst things is to have an accumulation of mud and dirt in a certain point, that will act as a sponge absorbing water when the car is driven in the rain, and maintaining it for a longer time in the garage
in my opinion you can not have the rust issue in your mind every time, you should relax a bit about that and assume that rust is inherent to a car as deterioration is to everything, including ourselves, you should try to enjoy the car as much as you can
said that, I will tell you, as strictly confidential matter, that I never drive my e9 in the rain, I search through the weatherforecast for a 0%-rain day to take it out from the garage
I have rust, I know exactly where, and I am reluctant to fix it because is a very exposed small area in the front wing, and I do not want to repaint the whole car !, so this is my choice, :-D
regards