You don't want your auto-top off to be connected straight to your RO/DI unit. If the top off system gets stuck in the "on" position for any reason it will lower your salinity, kill off anything in your tank, and flood your tank if you don't catch it soon enough. The safest way is to use a storage tank that you refill once a week or so.
Here's a top off unit I'm going to purchase soon, it's on sale also:
http://www.drsfostersmith.com/Product/Prod_Display.cfm?pcatid=13959&N=2004+113592