Phosphoric in 300 series stainless steel is generally okay, according the ASM Metals Handbook, Vol. 13 - Corrosion.
However, it does go on to say that any impurities, particularly chloride and sulfate, will accelerate pitting and crevice corrosion.
So, I will bet that those of you who have had pitting problems at the water line, or other etching, have high chlorides or sulfate in your water.
I never recommend storing anything wet. Don't care whether it's water, or Starsan or PBW or Onestep or vinegar. When you store metal items wet, you allow all the time necessary for oxygen deprivation of a local area to occur and then you have an activity difference and galvanic corrosion can start. Pitting at the water line is a prime example. Pitting at a dent or ding is another. Weldments are particularly vulnerable because the metal chemistry/structure is generally different.
You know how I store my kegs? Upside down and open. Sure I get a spider or two in there once in a while, but they are easily taken care of, and I just fill the keg with starsan and let it sanitize my racking cane and siphon.
Remember, I am just as FOS as the rest of you, but I hope this helps.
Cheers,
John