It's possible to keep a healthy reef tank using only tap but it can be more work and thing can go down hill fast. My little brother has only used tap with dechlorinator for almost 2 years. He still gets random algea out breaks but along as he stays on top of his water chages, the tanks has little problems. If you use tap then you should keep your bio-load (amount of fish) smaller, as the water will already be dirtier compaired to RO
I would suggest taking a sample of your tap water to your LFS
and ask if they will test your levels, Copper and Phosphates cause the biggest problem causers in tap.
If you are going reef then get an RO
asap your corals will thank you.
Here is picture of my my brothes healthy reef tank; using only a canfilter and 100% tap water.