A sealed empty keg is not sanitary after a while, according to an interview I heard with 5-star.
I do not buy it.
There is no way what after I sanitize and seal keg that this keg will become unsanitary.
How it would become unsanitary?
I clean, sanitize and seal my kegs till they are ready to be filled.
I have been doing that for about two years.
Do not leave starsan in plastic tubing.
Here I agree with tschmidlin.
By the way you said that you seal your keg with 5 psi of CO2.
If you leave StarSan in the keg, CO2 will get dissolved in StarSan and you will have a pressure drop.
I also do not know what impact would it have on StarSan because dissolved CO2 creates carbonic acid.