If all you are doing is replacing the slip-ons, you shouldn't need a re-map. However, if you also add an air cleaner upgrade you will need to re-map.
I had some occassional popping during downshifting when I first added by V&H Ovals, But when I added my V&H Dresser Duals, I also added the PCIII tuner and I don't have any popping anymore!
You'll love the Ovals!!!