If you live in an area where chloramines are used in municipal tap water, you definitely need to add Prime. The bonding of chlorine to a bromine allows the chlorine to remain in solution longer instead of dissipating out of water as a gas. That means that simply setting the water out in buckets for a day will not remove the chlorine from the new water.
Also, it's a good idea to temp match the new water to within +/-2 degrees of the running system to prevent temperature shock.