Yes, you do *NEED* an RO/DI unit for a reef aquarium. It's one of the most important pieces of equipment. However, you may be able to get by without one for a short amount of time, but eventually it will catch up to you once the hair algae starts to grow.
Tap water has "icky stuff" aka excess nutrients which causes you to have high phosphates and nitrates in your tank. Algae will feed off those excess nutrients and take over your tank and eat ur corals. You will notice many threads in this hobby about people trying to get rid of hair algae and using tap water is one of the primary causes of it.
I highly recommend getting one asap. FWIW, I went one year without one- it will vary for you depending on the quality of your tap water. But once it became a problem in my tank, I almost started losing corals- not pretty.
Make sure you are getting RO/DI and not just the RO unit You want "the best" water going into your tank. It helps if you get one with a TDS meter too so u know when to change the filters.
what gwen is saying is right on...
get the best you can afford.
here is the unit i have been using for more than 1.5 years (and many people i know use this unit) AQUA-SAFE-100GPD-MAXIMUS
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RO/DI Unit is one of the most important pieces of equipment in the reef hobby. If you can't afford on most local grocery stores carry units where you can fill 5 Gal bottles. Or you can purchase one on ebay, (like I did) just watch out for the crap. Mine was $120 Optima Automated Aquarium RODI system, from H20Science.com. I have been very happy with it.
For me, being tight on budget. I get an DI unit instead of both RO and DI unit. However with just only the DI unit the operation cost would be higher as I will need to change the resin in the DI unit more frequent.
IMHO, it would better to purchase the RODI system. The RO unit will remove most of the contaminant in the tap water while the DI unit will remove the rest.
In addition to removing phosphates and nitrates with a RO/DI system, you're also removing silicates, which dinoflaggelates feed on and can cause an algae bloom.
Scott... I am sure that you are happy with your RO/DI unit. As your experience has shown, buying water from your LFS doesn't hold a candle to your own RO/DI unit that you have contol over.