Sheesh! I don't know why they suggest that. FTM and Ancestry.com don't sync 100%. If you download a GEDCOM from Ancestry.com to FTM 2012, then you lose everything in FTM that didn't get exchanged with Ancestry.com: media notes, source notes, citation notes, citation detail text formatting, media categories, research notes, media file names, custom historical events, your task list, task list categories, custom reports, etc. I don't even know what happens with the Mills' based templates made in FTM then exported via Ancestry.com to GEDCOM. And what about a long standing public tree at Ancestry.com with member connections to other trees? What about all the comments? People who have downloaded media from that tree? History of records attached?
Tech support is either too cavalier or too ignorant in making this suggestion.