You should never do anything major without at least one backup of your files to your hard drives. While the probability is low, there is always the possibility that something could happen at Ancestry and years of your work disappear into the bit world. (I have three hard drives and considering a fourth, that I synchronize the data so each has disk has the same data files.)
While the sync is improving with each version of FTM, I would never trust all my data to the ability to transmit it through the internet and download it error free to my computer.
Even with out major change, is it not worth the risk of having no back of your data that is not in your complete control.