when you soak anything, including glass, in starsan for long periods of time, there can be a slimy residue that builds up. also starsan looses its effectiveness over time if its not made with pure water (like distilled or RO) due to it reacting with the minerals and stuff in the water. also an open container of it will have dust and other contaminants getting in, making this happen faster.
glass or inert materials are OK to soak. many metals (like caps) will dissolve or corrode over long periods of time.
there is no need to soak your bottles for longer than a few minutes. you will not gain anything by doing that, so its not worth the effort.