hi justin
in a nutshell NO you don't need a ro a di will do the job ok
the only drawback using di only is you will use more resin
using a ro will save money in the long run but as you are on a water meter this may well balance itself out as you live in a soft water area.
i myself used a di for the 1st year of going wfp but now use a ro as i was using 1 bag of resin per month
jay