SHIMANO Caliper mounting spacers neccessary?

You'll have to be more clear. What spacers are you talking about? Do you have a photo?

Are you taking about the the two washers that go on top of the caliper before the bolt head? Technically no but helpful if you brake tabs are perfectly faced. The allow for sligh misalignment.

Not quite right. Those cup and cone washers are necessary when you are using a post-mount to post-mount adapter between the brake caliper and the frame, because the adapter rotates the caliper slightly and thus puts the bolt heads out of plane with the caliper's tabs.

Here is an illustration:


But I still don't know if those are the "caliper mounting spacers" that janchondo was referring to, or if he was thinking of something else.

Better go tell pro team mechanics. Is it better to have them if you don’t have everything perfectly faces YES. Can it work without the and not perfectly faced brake tabs yes, but not optimal.

It's not a question of facing or not facing the tabs. It's about whether or not the adapter rotates/angles the caliper relative to the plane of the frame mounting tabs. Most post-to-post adapters (e.g. from PM160 to PM180) do. The photo you posted appears to be a non-standard adapter which does not rotate the caliper, thus the cup and cone washers are not required. You'll notice the team are not using standard-issue Shimano bolts on that caliper.

If you have them, no reason to remove them.

Thanks for the photos. You have used them correctly in the necessary places. They are needed there & should stay there.