Welllll....I think so. :p Hear me out.
Yes, fish can live in crappy conditions, but, to trot out and overused but still accurate phrase, it's the difference between surviving and thriving. After all, a betta can survive, sometimes for an extraordinarly long time, in a quart bowl with no heating that is cleaned once a month, whilst being fed goldfish flakes. However, we'd all agree that's no kind of life for the poor thing. Now, I know that's an extreme example, but water hardness is still important.
Soft water had a low mineral content, and soft water fish are designed to cope with that. When in soft water, hardwater fish don't get the minerals they require. The reverse is also true - when in hard water, softwater fish get minerals that they were never designed to absorb. This also effects pH, which effects osmoregulation (how fish absorb stuff).
That's a really basic explanation, but that's why it's important. :) If you do have the capacity to give each fish what it needs, then I would always do so. :) For a much, much better explanation than my garble, have a look at this super good article by Byron on TFK: http://www.tropicalfishkeeping.com/f...quarium-73276/