I needed gcc 6 for testing and decided to upgrade the whole system about three weeks ago. It was very easy and it's still pretty stable. I might do a fresh reinstall now...
If you go from stable to testing (or unstable), it should work relatively well closer to release time when bugs in upgrade path are mostly ironed out.
I was talking about the freeze itself though. It affects testing too which is normally rolling. So if the freeze is too long, things start becoming annoying.
