actually, for a liquid to be sold as "spring water", it must have the same chemical composition and quality as the natural water source. if it has added chemicals, then these additives must be indicated on the label. so long as you carefully read the product information and buy the same brand everytime, it should be fine. you would actually do best to buy the cheapest brand of spring water since it would have recieved the least amount of treatment in the bottling process.
in terms of simple money, it would be much cheaper to use a dechlorinator and tap water. any live plants in the tank would use ammonia compounds produced by the neutralization of chloramine. so you would be fine.
i used the bottled water method when i was at my internship. it was simply more convenient for me to not keep up with dechlorinator, measuring utensils, buckets, etc. i was keeping my puffer and khulii loach, as well as some newts, in the Wal-mart brand spring water. i did this for several months and am aware of no problems from it.
but you have to be careful to verify that the water is pure spring water. don't try to use floridated water or water with "minerals added for taste" (simply because you don't know exactly what they are).