Tim,
Just starting a tank, buying the jugs of water is not bad, especially if you have a small 10 gallon nano tank. For anything bigger, the cost becomes prohibitive.
This is not intended as a flame, but think about it for a minute. Getting water at around a $1 per gallon will set you back around $30 to fill a 30 gallon tank. For that $30 and for that amount of water, you are in the ball park for a Tap Water Purifier (TWP). Premium Aquatics has them on sale (or clearence) for around $19, excluding shipping. While a TWP isn't the greatest in the world, it does a passable job of deionizing the water. The water you get is DI.
For around $60 or so, you can get a 2 stage bare-bones RO unit. That too is pretty lean, but it clears a lot of junk out of the water. The more stages you have on an RO/DI unit, the better the water.
If money is a little tight, you may consider buying a TWP for now and getting a bare-bones RO down the road. Then you could hook up the RO unit and have the RO water pass through the TWP and get RO/DI. Aren't all of these letters fun??
It really depends on what you want to keep and on your wallet.
In closing, supermarket water is ok in a pinch, but you will soon get tired of lugging all of those juggs or water around.
HTH,
JohnD