The $apply function can run on any scope, including $rootScope.
Are there cases when it makes a difference if I run it on my local scope or if I run it on my $rootScope?
I'm asking because I'd like to create a helper function that wraps a given function in an $apply. To do that I'd always need to pass in a scope, which is A) annoying and B) not easy because I don't necessarily have a local scope.
I'd like to always have my helper function call $apply on the $rootScope, but not if there's some risk in doing that.
Running $apply on any scope always results in a $rootscope.$digest. The only case where it might make a difference is when you provide an expression as an argument to $apply. The expression will be evaluated in the current scope (vs. $rootScope), but afterwards $rootscope.$digest is always called.
The source code is quite clear: rootScope.js
Bottom line:
If you call $apply with no arguments, it makes no difference.