you don't have to cut them down, but as was stated, they become a bit unwieldy.
you don't lose much velocity going from 20" down to 16.5"
like 30 fps - that's within variance of ammunition lots.
so I consider it a no brainer...might as well keep the handling the same WITH suppressor on.
but I'm wierd, and am registering a 455 as an SBR to cut one down to 11.5"