How you get the best of both worlds (drinking good tasting water and getting ultra-pure H20 for your tank) is by adding a T-connection between the RO and DI units like so:
The T is a little hard to see behind my broken garbage disposal but you can see where the blue line comes out of the 4-point valve thingy and into the T rather than directly into the DI chamber. One half of the T goes into the DI (the blue line coming out of the T in the picture) while the other half goes into carbon "taste" filter (the white line).
On the "out" side of the DI chamber is a ball valve.
The result is that your drinking water by-passes the DI chamber entirely. You would get your tank water straight out of the DI from the ball valve without having to deal with the "taste" filter.
Another benefit is that you'll save your expensive DI resin for your tank and only your tank.