Can you use softened water in a dishwasher?

Yes.

When a dishwasher is plumbed into softened water you need to set the water hardness to 0 and you do not need to put salt in.

Most people find they can also reduce the amount of rinse aid they use.

According to Siemens product advice (number 08702400060) salt should not be added to a dishwasher if connected to a water softener.

Here is a letter from Bosch with their thoughts on soft water use in dishwashers: