We put plastic bushings mostly on 1" to maybe 1 1/4". On most anything 1 1/2" and up, it depends on what we are doing. We use metal bushings, grounding bushings where they are required. On anything 3" and 4" we use metal bushings period. Do you know how expensive it can get to lose a 2000 amp service because you wanted to save $20 on the pull box and $4 on a bushing?
I know of a contractor who blew up 2 services of 2000 amps because of this very reason. These were not short runs either, they were prabably 140ft runs each time, on the same service even. I bet that larger pull box and metal bushings were not near as expensive as the service repull on this. Too many times I have seen 500 kcmil pulled at an angle where it bends the plastic bushing to the point of breaking then the wire rubs and you get a fault. Come on someone, think about what you are doing before you try to save money.