I have grown out a few atf and when i do i cover the tank on all 3 sides. Back pane and both short sides blk out. I find it keeps them from getting spooked as much. I grew out 2 armys the same way. Worked out great so as far as covering 3 out of 4 sides i agree with it.
I came to the conclusion that when my pacu are in panic mode, they just can't see the end of the tank. It's open water to them until they slam into the end of the tank or slam into a tank lid. I needed something on the sides to be a big stop sign to my fish. The only thing I could think of was to cover up the sides.