I'm in Aberdeenshire, next to the biggest granite quarry in the country (Granite will not dissolve so our water is pretty pure). Our tap is rarely above 50 ppm. I recon your mate is full of Tosh, 5ppm indeed?
Never use your system above 0. If you do you run the risk of starting a job at 1 then by the time you are done it has shot up, meaning you have to do the whole job again, or worse, not noticing and getting some silly old bint phoning you up going mental.
What we do to save resin is have 2 D.I vessels in series, as soon as you get a reading of 1 move the last bottle back to nearest the tank and replace the resin in the other putting it nearest the tap.