Typically, RO/DI water is only used for marine set-ups or extra sensitive fish like Discus. It doesn't have minerals so therefore it has the water stats have a hard time staying constant. The most important thing for a fish is to have water that does not have swinging stats. If a change occurs, it is much better to happen over time... but when it changes quickly, the fishes body cannot keep up and it can actually kill fish in some cases.
If your not keeping especially delicate or marine animals, then there is really no need to use RO/DI for water changes.... especially since it is much cheaper to use tap water and just dechlorinate it.
Ox