If it's at all possible, avoid using softened water when watering your houseplants. Water softeners add a form of salt in the "softening" process, and these salts build up slowly in your houseplants' soil. Eventually the concentration can grow high enough to damage roots the same way that an accumulation of fertilizer salts would. You end up with brown tips and leaf margins [1] followed by stunted growth in general.

However, if softened water is your only choice, there is a way to minimize its negative effects. Hold your houseplants over your sink or laundry tub, then water heavily with lukewarm water, so it pours rapidly out the drain holes [2]. You will be adding salts from the softened water, but at the same time, you will be flushing salts out, so there's little or no net gain. This technique of watering obviously will take a little extra time, but when soft water is all that is available, it may be worth the inconvenience.