The TDS test will let you know if your RO/DI water needs to be maintenanced. But my personal opinion is this was an easy method for ppl to determine if they needed a WC. Start with a known starting point for TDS and then the additional TDS is from waste (or other additives).P.S.
It is probably more important that the TDS numbers not come from waste then their total value. Such as - if you have 180 TDS because your GH is 6, then that is better than water that starts at 120 with a lower GH then total lack of water changes has made it creep up to 180 due to waste.