Everybody who has an @mac.com email address also has the same @me.com address. It can't be disabled and they can both be used interchangeably.
I also only ever use my @mac.com address, but I don't know how you manage to inadvertently use the @me.com version sometimes. That's never happened to me. Is your @me.com address shown anywhere in the Mail settings? Saying 'here and there around the OS' isn't specific enough.
Also you don't state whether you are referring to Mac Mail or iOS Mail.
Currently Being ModeratedJun 26, 2012 10:58 AM (in response to Julian Wright)
One example of @me showing up is when I email photos from iPhoto ('11). It only shows @me as an option as the sending account, and not @mac (does show some other gmail and work accouns).
@me also shows up in the iCloud settings (via web browser), but I guess that's to be expected.