Activate any stat node you come across except Luck nodes. Unless the Luck node is Luck +4, do not activate it, Fortune Spheres are hard to come by and you shouldn't waste them on Luck +1 nodes. When you get Clear Spheres later, you can make the <Luck +4 nodes into Luck +4 nodes, and then you can use the Fortune Spheres. Other stat spheres are very easy to come by, so activate anything else whenever you come by it, they are easily upgraded.