Clean
Ever since 6.07 have initialized the system with the installer, and at least once a year.
Setup Assistant and CCC make it easy but even now I read on MacInTouch that basically "don't trust Migration Assistant."
Partition layouts change, wasn't that one of the things happens with Yosemite? And some lost access to Windows Boot Camp partition.
I would not upgrade without two backups, one of which is a safety net clone with the old working system and untouched.
Ever since 6.07 have initialized the system with the installer, and at least once a year.
Setup Assistant and CCC make it easy but even now I read on MacInTouch that basically "don't trust Migration Assistant."
Partition layouts change, wasn't that one of the things happens with Yosemite? And some lost access to Windows Boot Camp partition.
I would not upgrade without two backups, one of which is a safety net clone with the old working system and untouched.