The water is very rarely the problem and when you KNOW you have salt burn, even flushing with plain tap water is better than letting the plant sit there and scorch.

I've been growing with city water for years, I don't do a thing to it, and the only issue has been the with spring snow melt, the ppm of the source river seems to have dropped through the damned FLOOR and I ended up with a Ca deficiency... but it's all taken care of now...
If still in doubt, flush with distilled water.

Do you have the ability to test pH? That's very important.