It's not really an issue if you're paying attention. As long as it doesn't sit to air dry for days, generally rust shouldn't be a problem.

My issue with the mushrooms was that I made a kind of gravy with them after cooking the meal and the combination of the moisture and other stuff in the mushrooms plus the moisture from making it into essentially gravy, then leaving it in the pan for more than a day, did me in.

I just forgot about it after the meal, and didn't look at it for the next few days. It's entirely my fault that it happened. No question about that. Careless cooking habits can ruin cast iron pretty fast.

If you're coming mainly fatty foods in the pan, then there's not a lot of worry there, the natural fats will protect the surface, so you can be, more or less as lazy as you want; but with some foods, you really want to clean it as quick as you're able to prevent any damage to the coating/seasoning/nonstick polymerized layers that protect the pan.