I haven't done a bargello, but when I did the tube method I unstitched the row I needed to undo the tube. But, I needed the blocks the full size that I had originally sewn them together in.
Cutting seems like a waste, but if you are going to square up the outside edges anyway(on one pattern I saw, the seams were NOT lined up horizontally, they were all off by some variable amount for effect, and so the final had to be squared up all the way around, then one way or another you are snipping some bits of fabric away. Then, it makes sense to just go ahead and cut because the cut will happen later at that junction point anyway.