You WANT trace amounts of sanitizer in whatever you are using. If using a no-rinse/wet contact sanitizer like iodophor or starsan you should always sanitize fresh, and leave things wet. You should sanitize on bottling day (or brew day if you are brewing.) It only takes a few minutes. If you let a no-rinse, wet contact sanitizer like starsan or iodophor, dry your are reducing it's efficacy by half. If it is dry, any micro organisms that touch the surface render it no longer sanitized. If the walls are wet with sanitizer, that organism would be toast. But dry it would still be alive.
In fact because of that, the inventor of starsan worked it out so that starsan breaks down into "yeast food" in fact many of the compounds are found in most softdrinks, so they are perfectly safe when at the correct dillution.
There's a lot of great info in this thread, including the links to podcasts about the two most effective sanitizers we use, iodophor and starsan.
Sanitizer Question.
