Apple reserves selectors beginning with underscore for their own "private" methods and that would include properties. I don't think they reserve _ for ivar names though.
Personally, I would steer clear of using underscore to start any kind of variable name. It's an opaque convention. What if somebody else uses underscore for locals and no underscore for instance variables? What if you accidentally omit the underscore in a method where you have a local defined with the same name?
It's much better to make your local names different from your ivar names. For example in a setter you might use newName or neWValue.
