I think it depends on how often you access globals, and how deeply nested the globals are in the execution context(s). It would be quicker to access a variable from one execution context 'higher', than one from ten levels higher, for instance (depending on the JS engine, I imagine results and engine optimisations would vary).
I've modified your test a bit to have 50 accesses of the global variable, and the results are significant, ~5 times faster for local access in this particular test (for my brief tests in Firefox 19.0 and Chrome 26).
As with all performance related issues, rules of thumb can only get you so far. You have to benchmark your code and make optimisations that make sense for your code.