Several of the photos I uploaded at first to microstock sites were just "gathering dust" in my computer or backup CDs. One of them I particularly shot for a joke that I never carried out, and it has turned to be one of the best sellers.
OTOH, I see some sites have more or less constant sales, while others take "jumps" after several days - some of them months - of inactivity.
So my point is, what's the trouble of letting them sit around there, instead of your hard disk? I think they will eventually pay off, and you already went through selecting, keywording, etc...
My $0.02