Do pressure reducing valves also function as check/one-way valves?
First, no, they don't function as a check valve.  They limit inflow by dynamically changing the valve opening so that when less pressure is on the out side, the valve opens up fully, and too much pressure on the out side causes the valve to shut.  In a no pressure situation (where water could go backwards), it won't be impeded at all.

Assuming that this is for your residential plumbing, you generally wouldn't install a check valve, because you want to be able to drain the system at the shut-off if you ever need to do any modifications.