everyone's tap water levels are different...but i think when initally filling the tank, i think a high(er) level of phosphate, and other "bad" stuff could actually help jumpstart the cycle...then once it fully cycles use RO/DI...but for top off's make sure you use RO/DI ...you may get away with tap but after awhile you will build up toxins in your tank that will cause more trouble...think of it this way...
you take a 100 gall tank 1 gal/day evaps for 1 year(365 days)
you use tap water with "lets Say" 1 tablespoon of bad stuff(metal, phos, poop)/gallon
when the water evaps, water evaps...not salt, not metal, not phos...just water
so you dump a pitcher of water in every day
after 3 months of this you have ~+1/3 of a gallon of pure bad stuff in your tank(and your still adding more bad water to the tank), in that time you also do 4 x 20% water changes...your still going to be left with some bad stuff, but in that time your fish/corals/algea blooms are *thriving* in it.
after a year you end up with alot of bad stuff in your water...
take a garbage can and fill it with topoff-Ro water and have it sit around...maybe throw a Ph in it...it's much better off in the long run for your tank.