It may help to know how a RO
works first How a RO/DI unit works
Depending on your domestic water quality you may not NEED a RO
unit for a fish only setup or a fish only with live rock. If you try with out a RO
unit do you plan on mixing your own water or do you plan on buying saltwater thats premixed from a store? If you buy it premixed from a store how where you doing to do the daily/bidaily evaporation top offs. If you where planning on using water again from a store the time saved in getting a RO
unit will be well worth it.
You might find you have algae issues you just cant seem to get rid of when using tap water. If this is the case a RO
unit will most definatly help. If you dont have alge issues than you wont NEED a filter.
If you want reef its truly in your best interest to get one.