I have heard of direct plumbing RODI to either the sump, Display tank or ATO, and I think these are bad ideas. I think that the separate functions of making RODI, adding it to a Top off Reservoir, and adding water to the Display tank to maintain salinity are best if separate because each has possibilities for failure.
The RODI filter can produce bad water,
The valves that connect the steps can stick on or fail to open. This either because or, in spite of, the floats or level sensors.
The simplicity of using a float in the display tank to open a valve allowing flow from the water source through the RODI to the display has only two functioning components, the float and the valve. However, that simplicity is also a danger because if either of those fail you have major problems. Either your tank evaporates or overfills. An RODI system can produce dirty water. If that goes straight to your tank you might as well use tap. Plus there is no opportunity to dechlorinate the new water. (so maybe the RODI does that, I add Prime to each new batch of RODI that I make.)
If the float/sensor does not cause the valve to close on a direct to tank plumb the salinity goes down.
If the water level in the Tank goes down because a return pump goes off excess water is added.
If you control in the Sump and the drain is too slow the tank overfills.
If going to the ATO then that reservoir overfills. This does not directly affect the Display so it is better than the other case. Depending on where your ATO is located and what happens to the overflow you may still have a mess.
With direct fill from your source, the failure is not limited to a few gallons. When i am filling my make up Brute the water can only fall on my basement floor and go into the drain next to the barrels.
Once i have tested the new water coming from the filter for TDS, and am happy with the reading, I direct the flow to the Brute. Stagnant water can be dirty coming out of the RODI until it has flushed through. Once I have Primed the new water, I move a limited amount to my ATO reservoir. My reservoirs are 5 gal buckets. That is the most that a ATO failure can overfill my tanks. This amount would be 5% of my total; not good but my livestock will survive.
My ATO feeds the skimmer section of my sump. Two floats are in the return pump section. an on/off float and an overfill cut off; and a third is a low level in the reservoir. I am able to monitor the Sump level, the reservoir, and the Brute as I check for problems and surprises each day. Being involved keeps me engaged in a way that I wouldn't be if it was auto-matic. Also, since I am too new to understand or be able to justify the kind of redundancy that would make all of my arguments moot, I am good with it.
Whew I wore myself out. BTW is there a Captain Obvious badge or banner?
