It is beneficial to use the water BEFORE the softener. It's all usefull calcium and magnesium, and the softener replaces them with sodium. Besides, you can take the water's ppm into account when mixing your nutrient.

I don't use well water but I've used RO and a water softener. In both instances, I used a supplement called calmag.

I would say it depends on the particular softener and how drastic your water is in the first place.

I don't really like softened water personally so I shut mine off. Besides that, some softeners seem to add more sodium to the water than others.

I found that my well water, being hard, made it difficult to keep the pH low enough for my hydro cucumbers. If I used rain water in the hydro system, the pH was much easier to control.

You could try either way and see how it goes. The well water will be more likely to leave calcium deposites on the equipment.

The pre-mixed nutrient products usually have most of what they need so unless you have one formulated for hard water, you probably don't need the added calcium from the well.

Now that I'm into Aquaponics, the hard water is actually a good thing.

Here is a link that might be useful: My hydro page