Which side panels need to be accessible for maintenance?
In general, the equipment is under the side that has the topside keypad (button panel). That side is the most critical for access for items like the heater, spa pump(s), circuit board and so on. The main reason you would need to consider accessing under the cabinet on other sides would be something like a leak (extremely rare).
Note: All side panels are removable and are mostly for cosmetic purposes. There is a complete hot tub frame under the cabinet panels.