I would STRONGLY recommend NOT hooking up your RO/DI directly to your tank, regardless of the fact that you have an ATO system between it.
As Al said, if any part in this system malfunctions, it could lead to catastrophic, floor wrecking floods, which could eventually lead to your tank dropping through the floor or other good times. This scenario may not be likely, but you must keep in mind that every layer of complexity you add to a system increases the magnitude with witch it will fail. Always think of worst case scenario when planning things. Worst case scenario if you hook up a never ending source of water to your tank? The possibilities are never ending.. Worst case scenario with a smaller reservoir? You get the carpet a little wet.
Your reservoir doesn't have to be huge... it can be as little as 5 gallons, effectively using up an old salt bucket. Bear in mind though that if you have a big tank, you'll go through that pretty quickly.
Once I get my frag tank/refugium hybrid up, I plan on implementing
this simple, cheap ATO. All together will end up running me $20.