I use boiled filtered water. (.2 micron filter - don't ask me the brand - that is my husband's world & I stay out of it - only know he changes the filter by the calendar.)
In 2+ years it has been fine for me. But I only use the actual NeilMed saline packets - got some others once and they burned my sinuses - maybe not correctly buffered.
Going through the Simply Saline like crazy right now as I am fighting a cold.
Sue

Weighing in since I've been rinsing my nose 2x day for many years. I have used previously boiled water only, which I store in the fridge. Every 5 days I boil another batch. NeilMed specifically says to use boiled water.

My understanding about distilled water is that although the minerals have been removed from it, it is not necessarily sterile. The container may have been contaminated before filling. And once a gallon container is opened it could also become contaminated and therefore not sterile.