From the research i did when i was refitting our kitchen (where the softener is) i came across something that said that the water should be safe to drink (ie low enough salt), but because the softener never gets cleaned there is a risk of bacteria build up within the pipes which is why you should not drink the water (bit like drinking water from the tank in the loft but less chance of dead rats in it
).
Our kitchen tap and water dispenser on the fridge is straight off the mains but i have a separate softened cold water tap i use for kettle and coffee machine as that water is heated enough to kill off any potential nasties.
BTW watch out for how old your machine is, the resin filters only last a certain amount of time, we have an engineer come out and look at our last year (as it was not softening much) and concluded that resin was knackered and it cheaper just to buy a new machine than replace the resin. Ours is about 7 years old as FYI, but the engineer changed the slat flush cycle frequency to every 2 days (vs default 6) so its using a lot more salt but still softening. Buying salt from Costco is cheap enough to not worry about replacing the unit yet.