Matt, when it comes to SSP keep it simple. You only need one SSP. It should be the first thing in the assembly, in its own folder. It should also be the first thing in every part it's being used in (in a folder). By keeping it in a folder at the top of the tree these calculations are made first, in the background.
You can make many sketches in the SSP and many planes.
When you update the SSP you're updating all the parts and assy's it's used in rather than jumping back and forth between all the parts and assy's.