Well i don't think its really necessary. The whole purpose of doing that is so that the fish can get used to the new water, in other words making it room temperature. What i do suggest you do is to leave out the new water for at least 30minutes so that it will be room temperature. Also, when i bought my fish not to long ago, they told me i should poor the water that my fish came in with, in the tank with the new water. It makes the transition for the fish alot easier.
Hope that helps
Hope that helps