In the middle of my pickling I just had a thought.
Princess seams are interesting as they typically around the bust area at least have both a curve out and a curve in part, a fit-in-bigger-part that curves out (usually the side front panel) and a fit in, stretch-it-out-part (usually the centre front panel).
These can be a pain, the side fronts can pucker and pleat, and the centre front seam allowance always seems too short.
Notch the side front curve out part (stay stitch for security first just inside the seam line) and clip the curve in front panel along the same section.
This will make it all work.
The seam allowances in the front panel will now stretch comfortable to fit the side panel curves, which you can pin in without bunching because all the extra get in the way fabric in the seam allowance has been notched out.
If you are a visual learner, hold on.
My next project is a jacket with princess seams I think and I will take pictures.
In the meantime I hope the above is clearer than mud.