If it's a one time purchase, sure. If it's a monthly fee you can try it for a few months and weigh it against sales. If it makes sense financially, so be it.
I make sales on warning pages grouped with small lists. For a few minutes of extra effort it's worth it to submit to anyone who lists you and sends enough traffic, IMO.
KG Gary gives some good advice: every so often make it a point to drop the unproductive and add even more. When your list of freesites, and places you're submitting to starts to grow so will the sales.
