When I do buy an IC, it is around J4, and I place it in Manchuria.
By this point, I have a major tranny fleet that is full with units to bridge each round. The IC simply adds 3 more tanks to the westward sweep each round on Russia.
But in all honesty, unless Germany is starting to look a little over-taxed, I don;t build one. The 15 IPC’s for the IPC represents another loaded tranny build.
I simply tend to play Japan doing a massive initial outward sweep to increase IPC’s, then start a steadilly growing stream of forces marching on Russia. At first it is a soft breeze blowing west, but each round, the wind gets stronger until finally Russia is blown away by it (see, it is a strategy that even fits with Japan’s mythology :-) )