Don't worry about it. The sync option flushes any writes to disk immediately instead of caching, so you'll tend to get more writes if you're doing a lot of file manipulations which, over time, will wear out the flash memory on the drive. Fortunately, though, the write cycle life of most recent flash is in the 100's of thousands of operations before you should see any effects. If it's just an external storage key for moving files between machines, then it'll probably outlive you whether sync is switched on or not.
It's probably actually safer to have sync on, as there's less likely to be any corruption should you pull the drive out without unmounting it.