Tuning Rings: Yes! If you pull out, you must, must, must, must, always fill in the gap with a tuning ring. A narrow 2mm gap in the bore can cause significant problems. You should have as many rings of different thicknesses as it takes to to cover any situation. Have them matched to your barrel/body diameter.
Oooh. Multiquote DOES work here. I must make a note of that.I must be a musical Neanderthal, but I never witnessed that. I was never disturbed by the gap lurking up there. I just made a mental notice to get the longer barrel (the one sitting on my shelf), just in case.
The turbulences or whatever said 2mm are said to cause is not a myth, but acoustically insignificant.
Anyhow, I've never really noticed a difference. I was always under the impression that you used those things if a) you had a specific length you were pulling out and wanted to be consistent or b) you wanted to try to make the mouthpiece/barrel wobble less when you pull out.
I also fail to see how a gap between the barrel and the top joint would cause a problem. After all, the barrel seals at the tenon cork. Or am I missing something?