newbie - Is RO water really necessary?

I'm a newbie setting up a 55 gal fish only aquarium.  I have been 
planning to do a freshwater setup, but realized lately that the most 
colorful and interesting fish IMO are saltwater.  So, I've been looking 
at saltwater instead.  Still fish only, and flake eating fish only if 
possible.  Everything looks doable for a newbie like me, not too much 
more than fresh... except reverse osmosis / distilled water.

Is that RO/distilled really necessary?  That would add substantially 
more $$$ for those that don't have a RO setup, like me.

Thanks in advance