Nope, not necessary....only 2 reasons why you would ever need an r/o unit. #1 your pH is above 8.2 (after it has aged/settled). #2 you are trying to get them to breed and their eggs to hatch. Discus can and will thrive in just about any pH. Water hardness doesnt matter unless you are trying to breed them. I know several breeders who's tap water is 8.0 and have sucess with their pairs trying to spawn at that pH! Only thing that is necessary in a healthy discus tank are 3 things. Good filtration, warm water (mid 80's) and above all, good water quality...which is maintained through frequent water changes. If you can satisfy those 3 things, then you can keep discus with ease.
-Ryan