Well. It does not need to be rolled 180. However, I ran my phone compass app up next to the shaft/servo area of the heli and it gets up to 70 degree offset, some of it quite abrupt.
Moved the GPS back on the tail boom (And fixed the cracked support which should help on the vibrations) and I could not induce a Compass Variation Error on the bench, which I was able to do in it's previous location. Still appears off by 5-8 degrees compared to my phone,, but it's a consistent variation, so I'll take it.
There's also a compass offset tool in the Ctrl-F menu in Mission Planner, which appears to calculate the offset based on a previous log. I might just send it up, do a bunch of circles and then use that.
Compasses are so much easier in planes