You are now involved in the RO/DI versus tap debate, Marty..
This debate can get pretty intense, so I'll do my best to try to give you a rounded opinion. I am personally biased towards RO/DI, but as gotz_potential says, there is no Aquarium Bible that commands you to use it.
I would illustrate it this way: Many people enjoy owning Alaskan Husky dogs. Now, some people, when temperatures drop to subzero temperatures with wind chills outdoors, will say, "It's freezing out there! I'm taking my Husky inside so he doesn't have to deal with those conditions." Other people will say, "He's an Alaskan Husky. He can handle it," and will leave him outside year round. Neither of these ideas is necessarily wrong. A dog would probably rather be inside, but he will probably be OK outside as well.
So what you need to ask yourself is this: Am I the kind of fish owner who says, "I'd like my fish to have the cleanest water possible, devoid of anything that could cause a problem for them," or are you more like the one who says, "Fish are tough. It's true there is garbage in tap water, but they can handle it." Neither of these ideas is wrong in itself. You need to look at the pros and cons of both methods, and decide for yourself which one is best for you.
Having said that, I will tell you that as a new aquarist, you WILL run into problems here and there. I don't know anyone, on this forum or off, who has never made a mistake or had something unexpected come up. There WILL be times when something strange is happening in your tank, and you will be at a loss to know what's going on. When that happens, there are usually many variables to look at to diagnose the problem you are experiencing. This forum is invaluable for that. I guarantee you one of the questions you will get is what kind of water you are using. By using reverse osmosis water, you will be removing a variable from your aquarium. You will know that if something strange happens, there is nothing mysterious coming from your water supply. I have chosen to use it for consistency in quality, not because you will assuredly fail without it.
I hope I have given you a balanced viewpoint, pros and cons...I didn't give you a lot of pros for tap water, but I couldn't think of any pros other than that it is free...
