The compiler options, don't form an inheritance tree. Any option you enable (global/project/target) will be added in the compiler's command line.
So, just enable the option where it makes more sense to you.
For example, I never use global compiler options. I only use project/target options, even if it makes setting up the project initially a little harder (depends on project size of course), it's easier to maintain.
I know this isn't the best design and it's bound to change with the new compiler framework. We 've seen the shortcomings of some design decisions and they will change for the better