What grade of wire is required for the heating element on a J-Head extruder?
There are a number of things to consider:


Wire Gauge: a typical 40W, 12V heater draws around 3A. 24 AWG or lower would be appropriate (copper wire, CCA will require thicker gauge).
Insulation: the part of the wire close to the resistor leads might get too hot for conventional PVC installation. Consider using silicone, teflon or glass fiber insulation instead, especially near the heating element. If the resistor's leads are long enough they might stay cool enough for regular PVC insulation, but make sure you insulate the part of the leads closer to the resistor appropriately.
Solder: if close to the heater itself, the solder might melt. Consider using silver solder, or using a mechanical connection (ferrule/crimp) instead. Again, this might not be a concern if the leads of the resistor are long enough.
Flexibility: given that the extruder (probably) moves a lot, use flexible wire (stranded, not solid core) and provide strain relief (especially near the connections, and avoid creating too sharp bends).


Finally, resistors have fallen out of favor compared to ceramic heating elements. Since they are inexpensive and solve all of the above problems (the leads are already attached and appropriately insulated) consider using a ceramic heater instead.