Flat spots aren't such an issue, they'll occur relatively quickly (less surface area to wear down in the beginning, and that surface area carries more pressure because it's smaller). It's when they wear so far down that they only have half the diameter left that you should think about replacing them
(actually I'd start doing that bby 2/3rds just to be sure, even though the DEX210 has quite beefy drive pins compared to some competitors' cars).