I've heard mixed information on this.
I am replacing all my old ducting with new r6 flex duct. Continuous runs with no leaks.
Some recommend suspending it above the blown cellulose insulation, but I'm not sure I understand why I should do this. My reading suggests that the insulated ducts will sweat and get my insulation wet. I find this hard to believe but I see it mentioned a lot.
It will be much easier to blow insulation over the ducting, burying it.