It's been awhile since my exam, but you need a plastic bushing on any pipe where the conductors are larger than a #4 (I think). It would be easy enough to find out by going to the book.
I use PB's on all services. Bonding bushings of course are not required on pvc.
I seldom need to use bonding bushings in my work but I've needed them in the past on bigger jobs on conduits carrying a grounding electrode conductor (both ends must get bonded), or on seperately derived systems like transformers when you go to bond the neutral, or..... I could go on and on I suppose.....
We also used to use IMC sweeps on PVC UG pipe runs so the rope or pull string wouldn't cut into the PVC sweep and then you have to bond that sweep (obviously if you think about it) and a bonding bushing is the easiest method.
