In my own experience there are several sides to this...
First, if it tastes good, we (and fish) will eat what comes across our path.
Second, pelletized food, being dry and all that, basically, doesn't seem to go bad - I have never had a negative experience using older food, but then again, I vary their diet so widely that the "depreciated nutritional value" doesn't become a factor into the equation of things.
In other words, if you do vary their diet as you should, these older pellets, IF, and only IF, they are "depreciated in nut. value" will become their "potato chips" or "pretzels" - Snacks.