Make sure that your panels have an even copper distribution on each layer and between different layers. If needed add extra copper patterns inside the circuits, in between the PCB's or in the panel border.
Enhance copper distribution
Evenly distributed copper enhances the overall quality of the panel:
Better mechanical stability ( reduced risk for bow and twist )
well balanced copper plating in the PTH holes and on the copper pattern.
This means that when you design panels containing different PCB's you should only combine circuits with the same number of layers into the panel. For instance, never combine 4-layer, 2-layer and 0-layer PCB's in 1 panel. This will surely cause mechanical stability ( and a lot of other ) problems. Major problems are bow and twist, and differences in copper plating in the holes.