Switch mode PSU minimum load
Your calculations are correct. A 120 Ohm, 2W resistor is fine for the 12V rail. A 4W resistor might not get as hot if it is physically larger.

The spec sheet implies a minimum current for each of the 3 outputs, but you may get reliable operation with less load than this implies. For the 5A rail, you would need a 10 ohm, 10 watt load to keep it in spec, but the spec also relates to the regulation (+/- 1%) and you probably don't need such a load to keep the PSU turned on.

You need to apply a dummy load to account for the worst-case operating conditions, but can safely use a higher current load temporarily. If you want to experiment and save some power, I'd try ~220 Ohm on each rail, and see if that made it happy (but I'm guessing here).