Generally dissimilar metal corrosion is more of an issue with aluminum tanks, but it is on the other hand also somewhat self limiting as the outer layer of oxidation seals the metal under it. In a steel tank it is usually not much of a problem, but it is also not a self limiting process as rust tends to attract and hold more moisture, promoting more rust.
In either case mositure is normally needs to be present to help the transfer of ions from one to the other in the oxidation process.
The best prevention is obviously reasonably dry gasses and also using a thread lubricant as a barrier to minimize contact between the different metals in the tank and the valve and to protect the chrome plating on the valve threads.
Christolube is commonly used as it is O2 compatible and does not create issues with nitrox use or partial presure blending. You don't need a lot - an amount equal to the size of a pea is plenty - just distribute it fairly evenly on the first few threads of the valve and before you install it.
I'd also remove what oxidation you can from the tank neck threads using a wire brush. A suitable sized wire brush on an electric drill is probbaly one of the faster ways of removing any existing rust in the threads, but you want to be careful to ensure there are no rust or wire brush particles in the tank when you are done, so be sure to use air through a hose or small diameter metal tube to thoroughly blow out the inside of the tank while holding it up side down when you are done.
You also do not need to torque the bejeezus out of the valve - just a bit past hand tight is fine for a steel tank. More than that just increases the potential to damage the chrome on the threads.
