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I use 7X on flies #24 - #30, not so much because the fish are leader shy, but I feel 6X restricts the movement of the fly and causes "mini drag" situations. Given the breaking strength of modern tippet material, I don't really have breakage problems with 7X. Your point about scuffing tippet material is well taken. When I was first learning to flyfish, (circa 1954), one of the things I was taught was to rub the tippet in mud to take the shine off and sink it. Apparently, the obsession with floating tippets came along sometime later. I've never had a problem with a sinking tippet drawing even the smallest fly under.
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I've never heard of a knot being "stronger than the line" it was tied with".
For a knot to be stronger than the line it is tied with, the line tied with the knot at both ends would have to break at the same tension that the line alone breaks at. I don't know of a mono knot that does that. What most folks mean is that the knot does not slip and it does not pinch the line so as to weaken it very much.
Knots are all tied under tension and all knots depend on some form of pinching or friction caused by this line tension to keep the knot from slipping. I believe that weakens that portion of the line so that the knot may not slip but the line does break at less than it's rated strength. If you were to test a knot by tying a knot at both ends and pull until the line or knot breaks, I believe most of the breaks will occur at or near were the knot was tied. The breaks would not be randomly distributed along the line which would occur of the knot did not weaken the line at all.
I believe the very act of tying a knot causes a pinch point at the knot and I don't think the knot can therefore be stronger than the line itself.