lion_crazz No. But usually they are.
-Some people get RO water for free at their LFS.
-Some people use the API Tap water filter (small tanks)
-Some people buy RO water from their LFS because they do not need much
-Some people have great tap water (tested with a TDS meter to find out how much is really in their tap)
What are your alternatives to buying an RO unit? What size tank do you have?
Using RO water is not a necessity most of the time. But, it is very important most of the time. Few people have good enough tap water to not use the RO, without issues. Most unfiltered tap water will contribute to algae problems. If you have metals in your water, this can be a BIG problem for SWF. Aditiionally there are many other things that could be in it that you just are never really sure if its the cause when you start having problems. Using RO water eliminates all these. I ran my 65 reef for quite a while on unfiltered well water. But I had issues and wasnt sure if it wasthe cause, but couldnt rule it out either. So you are probably better off with RO water. There are alternatives as lion_craze stated, to buying one though. And if your going to have a small tank, then it will probably be a long time before buying a unit would justify it'self. However, if you going to be having a large tank, it wont take long for it to pay for itself.
Good Luck!