Took me a while to figure out the same thing on the masonry side. On a big old brick stack with offset flues, going top down just walks the crud down the shelf and it sits there like a little pile waiting to smoke back into the room. Pull bottom up and gravity is on your side the whole way. But flip it on a flexible stainless liner and bottom up can bunch the brush or snag a seam, so top down still wins there. The lesson is you match the tool and the direction to what the flue is made of, not to what some guy told you in 1985.

That "you're not making this any better for the finish op" line hit me hard when I heard it said to a coworker once. I used to think being a good machinist meant holding tenths everywhere, period. Took me a shamefully long time to see it’s about knowing where the tolerance actually matters. Now I treat roughing like its own job, get the metal off fast and leave enough for the heat treat to move around. Your lead hand did you a favor, even if it stung at the time.