Sometimes your water company can give a range of water hardness for your area if not there are test kits.

If you have a water hardness of 4 GPG you can benefit from a water softener.

Detergent bonds to the minerals in hard water in addition to the grease and food debris on your dishes.

The water softener sequesters these minerals freeing up all the detergent to actually clean your dishes.

Prevents mineral build-up/white residue on your dishes.

Since various state governments have begun phasing out phosphates in dishwasher detergent water softening has become more important.

Definitely check with your water company on the hardness level or check with a water softener company like Kinetico who can do a free test for you. US changed the dishwasher formulation to remove phosphates and softer water makes a big difference to getting a sparkling clean. A lot of people without a built-in water softerner will often use lemishine or something to counter the water hardness and increase the efficacy of the diswasher detergent. I also suggest posting on the appliances side where a lot of people have studied this and can recommend the best product for your machine etc.

Our old water softener wasnt working properly and we really noticed it with our dishes when it was replaced. Suddenly the glasses were looking much more sparkly and even the dishwasher door looked bright and new. Ours is set up so the water goes through the softener before it hits the dishwasher so we don't need extra in the dishwasher itself, but if water was straight from the well I would definitely use the softener in the dishwasher!
Alison

I have the F&P dish drawers with the water softener feature. I had it in our last house too.

We can make adjustment to the amount of salt that is used based on the hardness of the water. So, check out your manual, you may be able to do something similar after you know your water hardness.

We are still without a whole house water softener (grr), and I just ran out of my first order of salt. 7 boxes are what I ordered June 2012, and I just had to re-order. I held off, in case the whole house water softener was going to be installed first, (but it wasn't).

In our last house (hard water) when we went from a Bosch (w/o softener) to the F&P with the water softener built in, we really did notice that the dishes were cleaner and clearer glasses.

I buy the salt on Amazon, and usually just do cost analysis on what is the best combination to get me the best price.