Do I need a via or stitching cap when I transition between physical reference planes of the same potential?
The requirement for a stitching capacitor is covered in this diagram: -



This isn't quite the full story because decoupling caps from Vcc to GND are also needed (and will normally be present for the chips) at both ends of the signal. 

As far as I can tell, in the question, any signal layer is adjacent to a GND layer so this means stitching caps are not required. Stitching caps couple the signal return current between planes that are not at the same potential i.e. Vcc and GND. Both (usually) behave the same way for AC return currents.

Regards stitching vias, it is always good practice to couple multiple ground planes together at regular intervals so that ground return current paths do not form a significant "antenna area". In the picture above, if the Vcc layer were in fact another ground layer then it is advisable to have a stitching via close to where signal1 changes layers.

Having said all of that, plane to plane capacitance can be a significant reason for not using either method but, if the frequencies involved are high (50 MHz upwards I would say) then care must be exercised. Always check the natural resonant frequency of stitching capacitors - smaller values are better!