Should I reverse bias phototransistors?
Photodiodes are reverse biased to compress the space-charge region and reduce the junction capacitance.  This allows higher bandwidth.  There's no direct analogy to a phototransistor.  Usually, phototransistors will be slow compared to photodiodes due to long minority-carrier recombination times.  Light acts as the base current, so in an NPN phototransistor the collector would have a positive voltage applied, maybe through a resistive load, while the emitter could be grounded.