This question should be a sticky in the newbie forum. It seems that we are asked about spring water, distilled, tap etc every week.
As froggie said, it can be done, but it's not a good idea. As a new guy, you've got enough working against you. The one thing you have total control over is the quality of water you add to you tank, so every effort should be made to make it as good as possible. An RO/DI on e-bay for $100 will save you money and headachs in the long run.
Yes it can be done, but it shouldn't. I think your LFS guy is wrong.
chris