unless its got so much Chlorine in it that it sends you realing to the floor after drinking a big glass most plants aren't going to be bothered, especially trees, although some are touchy about the subject, Rain water off of non-asphault non-tarred rooves is preffered, then spring water, then distilled or RO water.

If your use tap water it is a good idea to let it sit overnight in the water can. That way the chlorine evaporates and the water is the temp of the bonsai. I have a little fish pond so I dip water out of that and then refill the pond. Zack

Even though I use R/O water because it is the least expensive for me, I would think that if you are dealing with only a couple of plants that you might go for steam distilled water. The best price is at WalMart where it is currently about 68 cents a gallon.
The reason why is that steam distilled is free of everything and gives you the option of controlling what nutrients you supply the plant!

I definitely recommend against tap water unless it comes from a spring and is only chlorinated. Hard tap water will eventually clog up soils with the metals in the water adhering to soil particles, the side of the pot, the rim of the pot, etc. It is the white crud you see on leaves and on pots in those areas where the water is hard.

If you are watering alot of plants then I definitely recommend that you acquire a R/O unit as the gallon per cost is significantly lower than bottled water from any store!