I'm on a well now, yay! But, when I was on city water I always squirted in as much water conditioner as necessary for my water change, then added the tap water directly to the tank. In almost 10 years of fishkeeping, I've never had a problem.
With just plants in the tank, there's not much point in doing water changes. Water changes are mainly done to reduce the ammonia/nitrite/nitrates built up in your tank from fish waste. How often you need to do them depends on a number of things...how high your fish load is, how much you're feeding, how well-cycled your tank is. I change water 2x (40-50%) per week in my planted discus tank. Other tanks are changed once per week.