Very few have gotten away with using tap water indefinitely and had desirable results. Sure it can be used with little or no apparent signs of problems but typically it will eventually catch up with you. A situation that happens that I don't think many are aware of is when evaporation happens, water is the only thing leaving the aquarium. All the undesirable disolved minerals, chemicals & compounds in our tap water stays in the aquarium. Each time you top off your aquarium with more tap water, these undesirable elements are being concentrated in the aquariums water. In other words, the somewhat safe, low levels of "bad stuff" in our tap water can get concentrated to much higher and often unsafe levels eventually, So you may not see a problem early on but in time it often becomes a problem. Compair this to taking a glass of water an putting it on your counter and letting it evaporate a little and keep toping it off with tap water. In time, because water is the only thing leaving the glass, the levels of minerals and other compounds will be so high that it would not be drinkable. The same thing can happen in your aquarium.
Using dematerialized (ro/di) water is your best option, it will significantly decrease the odds of algae and other potential problems in the future. The minor inconvenience is small by compared to the major loss that can occur due to water and environmental problems.
Also, using distilled water is risky due to how it was distilled. It is my understanding that copper tubing is often used in the distilling process and may leave trace amounts of copper in the water.
Joel