PJ’s comment is incorrect. See Why won’t my files upload from usb to ipad scriv - #3 by devinganger for a demonstration of how easy it is to recover a full Scrivener project from the Dropbox web interface. However, having said that, using the Dropbox software to sync the local save folder to Dropbox (and then to your other devices) is definitely the best and most trouble-free way to do it.
If you sync your live projects via Dropbox, do not use Dropbox to sync your backup files as well. That’s putting all your eggs in Dropbox’s basket. Many of us will use a second cloud sync provider (I use OneDrive) and set our backup folders to a folder that gets synced to that second cloud service. You have to set that up once per Scrivener installation and not override the backup location on a per-project basis (each project will use the general backup settings). It is also recommended that you increase the number of backups from the default (I use unlimited), set the backup to create the ZIP archive (so you only have a single file to sync/recover), and set the timestamp in the filename.