@Marxspawn
The location where you place the mod doesn't have any effect whatsoever on the set-bonus that you get for using that type of mod. As long as you have either 2 or 4 mods that confer a specific set-bonus, you get the bonus.
With that said, the slot you use for your primary stat is actually impactful, since you can only get certain primary stats in specific mod slots. For example, the arrow mod slot is currently the only one that offers speed as a primary stat, whereas 4 different mod slots offer health as a primary stat. So, you may decide that you want to prioritize a certain primary stat for a specific mod slot based on what stats are available for that slot.
Thanks again to everyone for the props!