To expand on my answer a bit, most RO units have a rejection rate of about 98-99%.
So even if your water has a high TDS it should bring it down to single figures.
If suposing your tap water is coming out at 400PPM then your RO should bring it down to about 008PPM, if it has only 95% rejection then your finished water is going to be 020PPM.
This will start using your DI resin a lot faster to bring it down to zero which is where you really want it to be.
Hope that makes sense.
Denzle