If your water comes straight from unpolluted lake or well water then you can go without it. If it comes from tapwater its like playing russian roullette. You may get away with it for a few times but not all the time.
happened to me before. I went by 6-8 months without using water conditioner ( broke high shool student ) and then one day all my fish slimed and gasped and finally died before my eyes. Big mistake. Huge, dumb mistake .
If you have ground water, the water company adds chlorine to the supply after everytime it rains so just to be safe I always wait a couple of days after it rains to do a change.