Is a protection diode needed for a resistive load?

If the load is a resistor you should be safe without a diode. Resistors have a parasitic inductance of some sort but the inductance value is usualy very, very small unless you are dealing with high power wire wound resistors that usually have a (somewhat) high parasitic inductance.

Have a look here too.

So the answer is no, you won't need it in almost all practical cases.

I know this has been answered already but thought I'd throw this in there as well--

You only need a diode on inductive loads, such as a transformer or a motor. This is because when the magnetic field collapses, it causes back-EMF that can damage your circuitry. The diode provides a safe path so that the EMF can dissipate without causing damage, but will only conduct when back-EMF is present.

Purely resistive loads do not generate any back-EMF, though in reality every load has some inductance and capacitance as well as resistance. However, for all intents and purposes it is generally safe to leave out the protection diode unless you have a significant inductance for the load.