Rubbish. I've bought a new dishwasher routinely in just about every house we've moved to in the last 30 years and I can assure you that dishwasher salt is not necessary.
The majority of dishwasher detergent tablets nowadays, including the cheap ones from Aldi, Lidl etc, all have what's known as a "salt function" and definitely do not require any additional salt to be added to the machine. I will accept that years ago, dishwashers did need dishwasher salt but this has not been the case for many years.
Keep your money in your pocket Valeriea - you will not need dishwasher salt. The capped hole in which you're meant to pour salt is just there for countries that don't have added salt in their dishwasher tablets. As hard as it is to believe, there are still some dishwasher tablets in some countries that don't contain the chemical that performs the salt function.
I'd be interested to know what woofgang estimates is the life of a dishwasher in the absence of dishwasher salt and any evidence in support of it.