Empty tanks aren’t necessarily a concern. I have tanks that had been stored for years without valves in them that pass both Hydro and VIP no problem. Worth noting, is that steel tanks are more susceptible to internal corrosion and sensitivity to moisture than aluminum ones, but all metal can and will corrode if not cared for. As far as 200-500PSI: Not necessary. 30-50 PSI is all you really need if you’re concerned about keeping contaminants out. Gas can’t naturally move from an area of lower pressure to an area of higher pressure, and water doesn’t diffuse across your closed valve and tank wall, so as long as there’s ANY positive pressure inside the tank, it won’t take on contaminants. I don’t recommend storing your tanks full for long periods of time becuase it’s hard on the valve seats. Accordingly, if there is water vapor condensation already present in the tank with the tank valve closed, this vapor cannot evaporate and escape. It’s really the latter that causes damage to tanks, rather than the amount of pressure stored in them. Higher pressures will also accelerate corrosion in the presence of catalysts / contaminants, due to the higher partial pressures of your primary oxidizing agent: Oxygen. If in doubt, remove the valve and let the tank air out until no condensation can be seen or felt inside, then replace it in a low humidity environment and have the tank filled from a reputable source.
Use your tanks often, get fills from a properly maintained compressor, and you shouldn’t have to worry much about this so long as you purge the valve before attaching it to a whip or regulator.