Please use this check box to allow the program to rotate layers while rendering mosaic.
Rotation Setup Button.

The speed button beside the check box will brings up Layer Rotation Setup:

Layer Rotation Setup.

You may set initial and final rotation angles and number of rotation steps in the appropriate text boxes. Speed buttons on the bottom help you to set a few rotation presets quick.

Below are few examples of rotation effect:
Layers with no rotation:
Layers with default rotation 0 / 360 / 12
Layers with no rotation.
Full rotation with 30 deg increment.
Layers with rotation -30 / 30 / 7
Rotation to 30 degs both sides with 10 deg increment.

"0 / 360 / 12" setting gives best results. With this setting the program reckons and compare all layer positions with rotation angle from 0 to 360 degrees. (Full circle). Overall mosaic looks is best but some of layers are placed upside down. If this is not appropriate, you may use not full circle rotation. The third picture shows layers rotated from -30 to 30 degrees with 10 degrees increment. It looks better than not rotated layers and all layers are in proper positions. They are just tilted and there are no upside down pictures.

More rotation steps means more rotation variants. Too many rotation variants may considerably increase time that is needed for mosaic rendering. For example if you set rotation increment to 1 degrees with full circle rotation program will have to calculate 360 times more layer position. It may takes 360 times more time to complete the rendering process.