??? Pix please.

It all depends....
On Addams family, they used the popbumper washers that are used for the ring & rod assembly,
as the support of the long green plastic piece on the extreme left side of the graveyard area.
I believe it has three locknuts to fasten the piece on top.
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1) No.

2) Sometimes. I know that doesn't help much but there you go.

Many folks have added the washers in between the post and playfield plastic to help protect the plastic from taking a direct hit on the corner causing a crack. Usually these are lexan washers but sometimes they are metal fender style.

The washers below the bottom of the posts are usually installed by the factory on newer games that didn't want issues with cracked/pooled clear coat.

In my experience pins from the early 90's and older had neither originally. I do like to add the lexan washers to certain plastics like slings and drop target banks. Especially for modern games that have multiball.

Sure they did. I've seen it in many games I've torn apart.

Here's one example on a BSD and I've done more than one and have seen these two in every one of them. I would imagine the design required a tad longer post and rather than having new ones manufactured, they simply used existing ones with a washer underneath.

What you as a restorer have to determine is whether those washers were installed originally or someone added them at some point. Usually it's pretty easy to tell.

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Not the same example of what I was describing. In relatively recent years, pooling/chipping of clear became prevalent. To counter or rectify this issue manufacturers started installing plastic/lexan/even metal washers under plastic playfield posts as pictured. I agree that elevated or structural posts and alike will often have flat washers installed beneath them.

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The issue of pooling and chipping under the post has nothing to do with the ball and everything to do with the clear and whether its fully cured or not. There are many assumptions as to why these issues occur. But in the end the result is a defect that is otherwise caused or amplified by tightening plastic playfield posts down to the playfield. One of the fixes if you will, from the manufacturers was to install the washers beneath the post to alleviate the pressure or as many folks believe, hide the defect. We could get into a big long discussion about curing times for clear and what not on these playfields but I'm sure you could find these discussions if you search.

I should add that there is always a chance any defect in the clear could get worse over time. Obviously these are not steadfast rules and every example is different with possibly unique circumstances.