I have the same dilemma as does everyone else with ceilings lower than 8ft.
Sure it's best not to have to cut them to make them fit, but after you have all the other corners covered (all 12 of them in a rectangle), you don't really have much choice. So yeah, I'd say cut them to fit if you want a the vertical corner bass traps to look neat and trim. If you don't want to cut them, you can also probably stagger them by hanging one high in the corner and place the other one on the floor with it's top 5 inches overlapping the top one. I don't think the gap between the trap edges and the wall will be of any concern. It won't look as good but if you will move in the future, and plan to use the traps elsewhere, it might be to your advantage to keep them whole. My 2 cents.