When driving a unipolar stepper as a bipolar with a chopper driver, should I decrease the current to prevent overheating?
The chopper will decrease the intensity for you, that's its all purpose.

The rate at what a magnetic field in a coil can change is V/L (voltage / inductance), to drive a stepper quickly you need to change the coils polarity fast and therefore you want to use high voltages (12Volts for a 3Volts motor for example). 

High voltages generate high currents (V = R * I), and the coils of your stepper should be able to dissipate then this much power:

P = V * I

That gives you a big P and dissipating power is bad because we want to generate torque and not heat, that means we need to lower I and the only way to lower I is by PWM the voltage.

The chopper does exactly that, chops (PWM) the 12 volts to prevent the coils from burning at the same time it drivers the motors with 12 volts (that gives you a nice polarity change rate in your coils)