I believe that the admonishment to not close off vents is primarily for the outlets on the furnace itself. But to maintain their legal CTA, they make it sound like any register restriction, even at the end of a 30+ foot run may cause problems. But it must be OK if the ductwork is folded over on itself or crimped through the framing. We put the adjustable vents in our rig back in 2010 and have not had a problem with it.
As I mentioned in an earlier post, we just had ours fixed as the ducting runs to the kitchen and living room were kinked in the underbelly and was not getting any heat.
Upstairs in the bathroom and bedroom the heat was blasting and it got so hot up there that you couldn't stand to be up there!
I even tried putting a rug over the bedroom vent and a waste basket on top of the bathroom vent to try and force some heat downstairs . . . but that didn't work, either.
Now that it has been fixed, we are now getting heat out of all four vents, but the flow doesn't seem very strong out of any of them now . . . but at least the heat seems to be even from all four vents.
I'm going to give them a good run tomorrow to see how well the repair is as I need to spend some time in the trailer checking out all of the rest of the repairs to see if they were done right (so far I'm not all that impressed with the fixes).