It helps eliminate hot spots. You don't need to pack every inch. Just use a loose insertion into all tubes. Trust me you want to do this. You don't want to find down the road it is blistering the paint in one spot.
Learning generally boils down to "Repetition or the avoidance of pain", some people learn by doing, some by watching and some just have to pee on the electric fence.
Whether you think that you can, or that you can't, you are usually right. Henry Ford
Not trying to hijack the thread here, but I was looking at the shopping list that came with my plans and when sourcing parts I found the same insulation in loose fill rather than batts (designed to spray into attic or crawl space). I was thinking of pouring it in to the tube then tapping the end shut with masking tape. The welder should burn right thru the tape easily. What you guys think?