Reset rotation by a key

While the rotate tool is active, press the UP ARROW key to reset the rotation for all the selected objects.

Reset rotation by the Rotation palette

To reset rotation of a single object, on the Rotation palette, click the center button, enter 0 and then press 8.

To reset rotation of more objects, click each one by one, and then on the Rotation palette, click the center button, enter 0 and then press 8.