I'm going to try the washer method when I install the rest of the posts. I think it's a valid idea. Here's my reasoning:
You want the posts to be secure, especially on a fast game like Quicksilver. This is why I countersunk my t-nuts, to get as many post threads as possible into the T-nut to make them solid. Some posts will take more of a bashing than others, especially the bottom post at the 4-bank.
My playfield has multiple Kruzman coats of clear. I think he put 8-10 coats on there. It cured for nearly a year. It's like glass & probably thicker than the standard Mirco coat. When I screwed a couple test posts into it snugly, the edge left a ring mark. Over time, the thick clear may start to ripple. More hits may result in the bubble cracking & chipping away. I saw this process happening on my TNA years ago. That's another similar fast game with a thick clearcoat.
So I'll add washers to the posts, so that the edge doesn't dig into the clear, but also ensure that the post is solid. The washer may sink a bit into the clear, but it will be under the post & not visible to the player. In theory, I should not see ripple effect around the posts after repeated plays & post bashings.
Perhaps I am overthinking it, but when I heard several people say they have used the washer method with good results, then I'm more inclined to try it.