CSS features that can degrade gracefully (E.g. border-radius) are set by default to adhere to the $graceful-usage-threshold variable. This variable defaults to 0.1 which means that when 0.1% of users (1 in 1,000) would be affected by removing the prefix for that feature, it will be removed.
This is one of the main reasons I continue to use Compass instead of SASS by itself. It’s a very smart system that derives its use threshold variables off of Can I Use data.
UPDATE: I use vanilla sass now days in combination with Autoprefixer.