I keep Star San in my glass carboys, but it's not because I think it's preserving sanitization or anything, it's more just a by-product of how I clean and store my carboys. After I empty a carboy of beer, I swish some hot water in it to get out the bulk of the yeast and trub, then fill it to the brim with hot Oxyclean solution and cover the top with a foil cap. A day later, I dump it, give it 2 rinses, then pour in some Star San and swish it around. This produces a lot of foam. I dump it out, and cover the top with a (sanitized) foil cap, but there are still copious Star San bubbles in the carboy. I set it on the shelf, and eventually, those bubbles settle out, and I'm left with a small puddle of Star San in my carboy. I don't bother dumping it, I just leave it there till brew day, then swish it up a little more (to re-sanitize the interior of the carboy), then dump it out immediately prior to racking chilled wort into it.