Jamil et al...
You mentioned being concerned about Star san residue in your kegs affecting our head retention.
Why not just drain and dry the kegs upside-down? Pasteur proved that microbes cannot migrate UP without assistance, and assuming your area is wind-free, this should be enough protection to prevent contaminants. Besides, the beer should have enough alcohol to hamper other microbial growth anyway, no?
Good question. I do use Star San in my kegs. I do worry about the amount of Star San in the keg affecting head retention, so I invert the keg and drain out any liquid. I don't bother letting it dry, as I think the reside would still be there and I don't think it makes a lot of difference.
All in all, Star San is an excellent product and I wouldn't hesitate to use it. I just make sure I dump the couple ounces that collects in the kegs before use.