So, this is an interesting thread. Now I'm reconsidering what I've been doing.
I usually clean several kegs at a time. Then either later that day or the next, I fill one with StarSan and purge with CO2 into the next keg. I continue until all the kegs have been sanitized and purged. They're pressurized and I store them with plastic bags over the tops to keep dust and such from collecting on the posts.
When I need to keg something, I grab a keg, check the pressure to ensure that it's tight and good to use, then I spray StarSan on the posts and start transferring. They were clean and sanitized when I put them away, so if they're still holding pressure, I assume they're still sanitized inside.
This seems reasonable to me because of the StarSan and the CO2 atmosphere inside the kegs. However, I've never considered the possibility of mold. I know that mold can survive in low pH environments, but I don't know if it needs oxygen. I wouldn't think that it could grow in a small puddle of StarSan with no other nutrients, but
@Agent seems to indicate that it's possible. Hmmm...