No, Warsaw should not be changed to reflect is state before the destruction brought about by the Nazis. Change is inevitable worthy in the wake of destruction, the city will never be the same as it was before and at this point nearly a century of new history has been created with the city on its present state. It would be better to spend restoration resources on essential services or infrastructure maintenance.
Warsaw can never go back to the city that is was once was before World War II. The city that it is today is Warsaw. It's history, it's architecture, it's culture is what makes up a city. By trying to go back to the 'original' state it discounts all the movement it has made since World War II. Incorporate the artistic 'improvements' as part of the city's history and move forward.
Building the city based on the artist's depiction of the city adds a unique element of history to the city of Warsaw. It is in the nature of art itself to express a personal interpretation and vision of what you see or feel. The artists painted the city as he saw it, or felt it to be rather than exactly replicating its appearance, The fact that the city has been rebuilt to match this interpretation adds a very unique element to Warsaw's history that should be celebrated.
There is no reason to change things back to the way they were just because of historical accuracy. If people are happy with the way things are now, there is no reason to spend all that time and money. Cities change over time, and it is rediculous to exptect them to stay the same forever.