Hi! The Ninja Tool Essence you get from Ultimate Training are quite valuable as it offers a very high value for specific stats. What I would personally do is find items you know you would use for a long duration such as Gold or Red Ninja Tools, and use the Ninja Tools Essence to re-forge the stats into the Ninja Tools. There has been quite a number of threads talking about this and some players looking towards the future would decide to save these Ninja Tool Essence for specifically the Red Ninja Tools.

Hope that helps =p

~Jib