It really depends on your location. here in the UK our tap water varies greatly from different parts of the country.
The problem with tap water is that it usually carries phosphates and that is bad news for a marine setup as it will spring forth hair algea in abundance.
RO_ water is far better as all these nutrients are removed. Also, our water authorities periodically put chemicals into the water to kill off freshwater shrimps and creatures that live in the pipes etc. if you happen to fill you tank with water that has been treated with this then you can kiss you shrimps, inverts and probably your fish goodbye. RO is by fare the best.
I use Natural seawater (as its good quality where i live and redaily available) but i top up with Ro water to keep the system stable.