It is always better to have too much water than too little while hiking. However with experience you learn how much water you need in various terrain and weather conditions. It is not wise to carry too much water if you can go without less and having extra liters of weight that you will not need to use. This results to unnecessary energy waste. It is also to know that the exception is true when you hiking a terrain that you are not familiar with or that you are not confident about the quality of the water available. Having an extra 2 liters of water available apart from the norm should be sufficient.