Should all unused analog input ports be tied to ground for accurate A2D measurements?
Any floating pin will act as an antenna, and may interfere with values around it, this is one of the major disadvantages of using analog.

To get the most accurate readings with minimal amount of interference, you can pull all unused inputs to ground.

The accuracy will not change, as the accuracy is determined by the sensor itself. However it will reduce the amount of noise / interference, which will give you more accurate readings.

These pull to grounds can be accomplished by configuring the unused analog pins as digital outputs, and setting them low.

pinMode(A1, OUTPUT);
digitalWrite(A1, LOW);