I usually have a tag on my site that says "credit where credit's due, folks!". If people wish to credit they can. It doesn't bug me if they don't. Plus I make a habit of tagging on my name on sheets I like to keep as "mine" on my site. I don't find much redirtribution of sprite sheets on the net, so it's unlikely people'll come and steal them out from under you and claim it as their own.
You know tho .... back was spriting was young and few people did it, they demanded credit for their rips. Very few really did full-motion game rips or even really knew how to rip at all. But once quite a few people started doing it crediting for just rips began to get redundant. I mean .... two people rip the same game. It happens.
It's only natural to feel a bit possessive of rips you ripped, but asking for credit on use is a pain. Really, do you even want a bunch of people e-mailing you for permission? Nah.