Discovered something quite interesting!
I first noticed that when Rocket was enabled, a clip in Source Monitor played very smoothly (using Rocket and CUDA), but when attempting playback in Program Monitor the image remained in freeze frame while timecode advanced. And this happened even when the smoothly playing clip from Source Monitor was on the timeline and tried to play in Program Monitor. Disabling Rocket in Source Settings resulted in poor playback in Source Monitor, but smooth playback in Program Monitor.
I got to thinking what could be the cause of this -- and noticed that I had set Source Monitor playback resolution to Full, while Program Monitor playback was set to 1/2. So I set both to 1/2 playback resolution and enabled Rocket. Playback in both monitors was abysmal.
Next I set both monitors for Full playback resolution... and Hey Presto! Smooth playback in both monitors, using Rocket and CUDA!
So for some odd reason, my Premiere installation only uses Rocket when playback resolution is set to Full.