Hi Pat,
From what I understand and i'm no expert. But it's wise to collect rainwater and use that, which makes sense I suppose.
Although I must be honest, I've been lazy and haven't got around to that yet, from what I've researched, hard water can result in more serious damage if used exclusively over a long time, however infrequent watering with hard water will not be the end of the world.
Again though, no expert here and I can't speak for junipers (which by the way, I love the look of yours!)