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How necessary is it really to wash off plastic parts with soap and water before you prime them? Common wisdom says do it, so I've always done it. But is it really necessary? Anecdotally it seems that washing may really only be required for resin (finecast) type parts.

If it looks shiny, definitely. If you're airbrushing, again definitely. If it's expensive, yes.

For plastic, my experience is it's totally unnecessary. For Forgeworld though it's an absolute requirement. I don't remember if I washed my only finecast models, I think I did. Can't hurt.

I wash it all because it's either necessary such as with resin, or very quick and easy as with plastic sprues.

I've never washed plastics in 25 years - no problem. That said I do use a good primer (not GW spray paint!) so that may have something to do with it? I use Vallejo polyurethane primer through an AB btw.

I do wash anything that I have stripped (to ensure the stripping agent has fully gone) and all resins.

I have never washed plastic and never had a problem.

I failed to wash resin once and it was a nightmare.

I've never had any problems with unwashed plastic. I did wash a finescat model once, but I'm not to sure about what the effect was.

I have chanced leaving FW models unwashed before and got away with it, but in general it's a good idea to wash anything where release agent has been used - Resin and Metal.

In my case plastic GW and other generaly not necassery.
You might want to lightly sand really smooth surfaces though as some paint and primer might not stick to really smooth surfaces and can come pealing off later. (This happened a couple of times with me mostly around the edges.)
Metal never gave me more problems then plastic.

Resin like FW I recommend hot not boiling water and some dishwash soap and leave it soak overnight. If necassery repeat but use a soft scrubbing brush on the parts that give you trouble. Be careful as not to damage you parts.

Here's the thing: plastic injection molded parts (you're regular Citadel miniatures) don't need to be washed unless they are dirty. It DOES help get rid of dust and debris, but there is no long sticking release agent due to the PIM process.

Cast resin (Forge World and FineCast) use a different material, the resin, that will stick to the mold, so the before casting the molds are coated in a release agent. This agent, if you do not wash it off, will be between your base coat of paint and your model. If you don't wash it off, some time down the road your paint will start coming off.