Is watering a new seed lawn and garden with softened water would be good or bad?

Q: Softened water in the garden good or bad?

A: Neither good nor bad.  Read more below…

Most water softener installations include a system bypass for water pipes that lead to outdoor faucets so that you are not using softened water in gardens, etc.  You should be able to see a pipe that connects to outdoor faucets on the main pipe leading from the well or meter to the water conditioner; this bypass pipe usually has its own shut-off valve as well.  If your system installer failed to provide this bypass, any competent plumber can add this for you.

In the case of no bypass, or if you are watering with a can from the kitchen faucet, softened water poses little risk.  Softened water has had its calcium removed in the ion exchange tank and replaced with either sodium or potassium, depending on which salt you are using.  If you have a water conditioner, then most chemicals (e.g. chlorine) and other dissolved solids are removed as well.

Softened/conditioned water is very similar to rain water (essentially distilled water) with the exception of the added sodium or potassium.  It is conceivable that sodium buildup could harm some plants, but this is very unlikely if you are watering sporadically.

Ideally, you would switch to potassium, which is better for human health as well as plant health.