Distilled water can be fine for water changes it can also be a poor choice, same goes with RO water or RO/DI. But these choices are far superior to tap or well water that has been untreated.
Its hard to give you an answer because it all depends on how it was processed or distilled....ie....how old are the filters and how was is bottled, boiled, cooled.
So I think your LFS was feeding you to some extent a line of bull to get you to purchase their water.
Thomas