I've done quite a bit of corrective work for drywall, noticing wherever there seems to be cracking at inner/vertical corners, it's almost always due to lack of stud support.
But when boxing around basement ducts, say, on the upper horizontal corner, things seems less standard. Some will add blocking at the top wallside and some won't. This is almost like what happens around the bottom edge of showers where the horizontal block is "absent" (but in showers, it's the lower edge and not an inner top corner like directly above it that abuts a ceiling).
The concern is cracking/potential inner corner joint failure over time. So what is the verdict - to block or not block the tops of walls that have covered existing plates - where boxing is being added around ductwork
Then there is the added element of: what about when the wall has been drywalled prior to the vent boxing? Cut out drywall and block Or simply frame the vent on top of the existing wallboard and then tape
