I have all of them locked to the panel, but they keep moving around upon logging out and back in. So, for example, sometimes the Indicator Applet Session is farthest to the right, but sometimes the Clock/Calendar is, and occasionally the Notification Area is. This is a small issue, but annoying.
There is no real solution except to wait for bug #44082 to be fixed. However, there is a (very hackish) workaround. Put your panel applets where you want them, then run
I use a simpler and more user friendly solution, IMO, than dumping to XML and restoring (which didn't always work for me, btw).
So, the answer to life, universe and everything is (not 42):
Whenever I want to make changes to the panels I disable the setting, make the changes and enable it again. Worked like a charm for me so I hope it helps others too.