What are opinions on using blown cellulose or fiberglass on the ceiling of a vaulted scissor truss(raised heel of course)? Is there a certain pitch that this method needs to be avoided?

Are you including stabilized and dense pack cellulose? They work even when vertical.

Good question. I was actually referring to dry, loose fill as it can be done with standard rental equipment and DIY. I suppose I should ask the insulation contractor as they might be able to do it cheaper than me with their stabilized/dense pack equipment. You think too much settling risk with the loose fill?

Even stabilized goods can settle somewhat in a vented attic in a climate with high outdoor dew points, but at least it isn't moved around by high winds coming through the soffits, and it won't slide down a sloped ceiling with time/wind.

Low density cellulose has better air-retardency than low density fiberglass, retaining it's thermal value over a large temperature range (low density fg is lousy un sub-zero F winter contitions, and issomewhat translucent to radiated heat under hot roof decks.)

With any fiber insulation you'll still need to leave a vent-space between the roof deck & fiber to avoid seasonal moisture issues.

What pitch?

Once it settles out, it is pretty stable and forms a somewhat locked in structure.

The code up here north of the border only allows blown in for ceilings less than 3/12.

National Fiber has a paper on this, I believe they allow their product up to 6/12?

Personally I've only gone up to 4/12. Last 8/12 vault I discussed it with an insulator and we determined it to be way too risky. However we've upgraded insultion in 6/12 scissor trusses where we insulate it from the attic and it does do fine.