WOW!!! I would question her and maybe think twice about buying stuff from her!! It is not necessary but the water will need to be treated for chlorine and other hard chemicals. RO water takes out 99% of the bad stuff and leaves you pure filtered water. Then I mix mine and get up to temperature and make sure all my cal, alk, PH, and Salinity is where they should be and it all mixes for a few days before I ever add water.
I can not believe a LFS just dumps tap water in her tanks. She must have tons of algae and un happy fish and corals. If I were you I would educate yourself and go in and slam her with cold hard facts and she what she has to say!!