Yeah I have done it before too....it was so easy to just throw the hose in the tank and wait for her to fill up....HOWEVER....I think it's worth starting with RO/DI. Both large systems that I have done this with had micro algae problems that lasted well beyond the cyano cycle period....whether it was cause of the tap water or not is another story I guess. But by starting with good RO/DI I could have ruled out the water if I still had micro algae problems.

But....that's just my experience. Up to you.