As I'm sure you well know, a mixing includes vendor specific prefixes. And the whole point of mixins is to make it easier to maintain. Once they're written, you don't need to worry.
The percentage of browsers that still require these prefixes are miniscule. I've been using border-radius, box-shadow, box-sizing, and others without prefixes for a while now without issue. Check caniuse.com for their status and you'll find vendor prefixes are dropping like flies.
Unless you're supporting Firefox 3.5 for some weird reason, you haven't needed vendor prefixes for almost two years.
That's great news if it's true. Happen to have a link or citation backing that up??