I don't I used the one that came that was a mixed cartridge on the 90 per day system and then started using just the di after that havent noticed a difference. I did pick up some of the di from Bush but haven't tried it yet but have heard good things about it.
Just another thing to think about for DI, don't get mixed bed media. You can keep the cation and anion media separate to make it easy to recharge later. If you want to go that route.