Dishwasher water supply line has leak where it enters the metal shutoff valve (at the back side of the metal nut - around the plastic line). Does this nut work with a compression sleeve and may need a twist with a wrench or are these plastic pipe to metal valve joints
A permanent creation that I need to replace altogether

It depends... D/W like Bosh have integrated plastic water supply that came installed with D/W. It has cone-like shaped end that replaces compression sleeve. If the tube was installed by local plumber than it probably has metal insert tube with plastic sleeve and metal nut.

To answer you question : yes, you can try to tighten it up. Give it a 1/4 turn. Don't overtighten thought. If it still leaks, give it one more 1/4 turn...

Many thanks Milo,
Your answer reads as though you're referring to the line at the D/W inlet - my leak is just off the supply line under the sink, at the shutoff valve inlet before exiting the shutoff valve into a copper line that I'm confident has a compression fitting - I guess my real question is, "Is the inlet to the metal shutoff valve - where the PVC water supply line is inserted - a compression fitting? If this connection is glued or permenent, I don't want to wrench the nut and break the bond or the pvc pipe? Thanks again.