I agree, for HPR tubing, a backer is not really needed, unless the cuts are really long and the tube deflects once several of the slots are cut. But for Estes style tubes, I used a spare coupler to 'back up' the cut. And using several light passes seems to also help reduce tearing/ragged edges.I would say a coupler is good advice for standard tubes, like Estes, which are thin walled. For LOC tubes or Blue Tube, not required. I recently cut fin slots in a LOC airframe without a coupler, no issues.
I do use the gold X-Acto blades as they are MUCH sharper, last much longer and are worth the extra cost for stuff like this. I cut all four ring tubes for my King Kraken with just one of those blades. I use regular X-Acto blades for less exacting tasks, like cutting out balsa fins.
I suppose you could soak a coupler with CA and then sand it smooth to make a better backer than a plain paper tube.