The only time I requeried was to a former assistant agent who wrote a half page of comments to my WIP when he and the primary agent rejected it. When the assistant became a full agent, I asked if he would consider taking another look (I had reworked quite a bit of the WIP by then.) He said "sure" and I sent it off again. He still didn't love it enough to pick it up but it was a good experience. Had we not exchanged a number of emails over the first submission, though, I doubt I would have requeried him.