There is no way that all of the water is draining out. Maybe if it was left tilted for hours it might drain completely, but the location of the valve, being so close to one leg, makes it impossible to leave it tilted.
If the valve was centered between 2 legs you could easily prop the unit in the tilted position, but the way they designed the valve location makes that nearly impossible.
So, we drain what we reasonably can, then set the unit upright and some water is going to run back to the bottom of the tank. Even if the valve is left open, I doubt there is enough air circulation for the remaining water to evaporate before the next daily use.
It seems to me that the bottom of the tank is always going to be wet and therefore subject to rusting. Sure, less water means less surface area to rust, but if the very bottom of the tank is "under water" 100% of the time, 1/16" or 1/2" isn't going to make much difference at the very bottom, is it?
*If* the tank is going to explode due to rust, which is more dangerous:
Having a small area (the very bottom) that rusts out and blows or letting a larger area rust, spreading the release point over a larger area? Sure, it will probably still be a single point of failure, but if the area around the failure point is already weakened by rust, wouldn't the explosion be less violent?