Any time you want. Use especially after heavy use and before mid to long term storage. Many oils will provide good corrosion resistance in a typical gun storage environment. However, with long term storage, there is the chance of the oil running off or oxidizing. Wax is a solid and will stay where you put it except in cases of high heat.
Caution - excessive rubbing on case color (and blue to a lesser extent), especially with a dirty (think very fine grit) rag, can abrade (think remove) the surface.
