I agree, and that's certainly the safest - perhaps I'm being unwise in syncing. But the way I work in both Ancestry and FTM makes it very frustrating if I don't.
Unfortunately I think the only way to be fairly sure of what a sync has done is to export GEDCOMs from both FTM and Ancestry before and after the sync, and compare them, but even minor changes can renumber source references etc., and that proliferates through the gedcom, making a straight comparison very difficult. I just rely on keeping a history of FTM backups for as far back as I can, and treating FTM as the "master" that I can restore and even re-upload if I have to, because I don't want to lose FTM-specific things such as resolved locations, colour coding and tasks.
Hopefully not much longer to wait! :-)
Unfortunately I think the only way to be fairly sure of what a sync has done is to export GEDCOMs from both FTM and Ancestry before and after the sync, and compare them, but even minor changes can renumber source references etc., and that proliferates through the gedcom, making a straight comparison very difficult. I just rely on keeping a history of FTM backups for as far back as I can, and treating FTM as the "master" that I can restore and even re-upload if I have to, because I don't want to lose FTM-specific things such as resolved locations, colour coding and tasks.
Hopefully not much longer to wait! :-)