I've been lurking here for quite some time and as I haven't seen anyone address this issue specifically when it was something that I was looking for solutions to ... so I thought I'd share what I have found that seems to work well for me. I have found that a big part of top-down design is the ability to drive shared properties and equations across multiple parts. Without this there is limited parametric capability in the design. I had a hard time figuring out exactly how to do that as external files added another level of complexity ... yet another place to go to manage the part. Placing them in the assembly level was causing circular rebuilds and failures to update consistently. Having external references flow according to the core tenets of SSP should also apply to Global Variables. The setup I now use took some digging to discover, but seems to be flawless and follows the SSP methodology very well. If anyone has another method that works well, or reasons why this isn't ideal, please let me know. My current method is to place all Global Variables in the SSP and reference them directly from within each part just as I would any plane or sketch reference. The following example uses Global Variables to control and account for material thickness offsets at part intersections and overlays. 

Below is the top level assembly tree with 3 SSP parts having -000 at the end denoting them for easy reference. All Global Variables are located in the primary SSP A00020141-000.

The A00020141-000 SSP is in every subassembly. The Global Variables listed here control a handful of different material thicknesses across the entire assembly and drive many of the equations in the sketches and planes of the SSP.

The image below shows the part A00024415 as a component within one of the subassemblies A00020142 which also contains the SSP A00020141-000 with the defining planes, sketches and data including the Global Variables. Note that A00020142-000 is a secondary SSP that contains definitions specific to this subassembly that do not pertain to any others.

This image shows the Equations within the part A00024415 referencing the Global Variable directly from the primary SSP. Note "D1@Sketch2" and "D2@Sketch2" below are used in the second image below.

This equation is then used to control an offset definition plane and sketch dimension offsets from planes defined in the SSP.

One of the great things about this setup is that the references to the Global Variables in the SSP are the same everywhere regardless of where they are used because the path to the SSP is the same regardless of where you are in the assembly tree.