Simple Test:
Put your tap water in a glass jar.
Set it in a warm, sunny windowsill for a month. (Around here that's not going to be possible until spring.)
If the water turns green and icky without having added anything to it, you may want to reconsider using tap. Most people will get green water from this experiment. The brighter the light, the more impressive. I left some freshwater plants in a container on the patio last summer, and I ended up with a few plants and alot of green foam. Some places have high quality well water or good procesing and will be fine.
As for yas2903's contention that he would have to do a 100% water change, I'd rather do a slow change. Sure it's going to take forever to get all the trace elements that came from the tap water out of your tank, but as long as nothing was dying from it, what's the hurry? Excess nutrients will get absorbed by any algae you have, then after passing through some fish it will get pulled out by your protien skimmer. If I obtained an RO/DI unit after having filled the tank with tap, I'd just use a normal ammount of RO/DI for topoffs and routine water changes.
It's almost always better to do everything gradually in aquariums, unless you're trying to solve an extreme crisis.