Well, assuming you're putting it in a box, a box has six sides. So you're saying the bottom and top are solid and three of the for sides are solid? Heat rises, so ideally you would have ventilation above and the open side would be the side with the finned pipe coming from the pump to the tank.
But really, it depends on how you use it. It's rated at 50% duty cycle, but I've never pushed mine that far. I only use it for a few brads or pins here or there. In that sort of usage, the pump might come on once every 5-10 mins and wouldn't get hot enough to be concerned about getting placed in a cabinet. If you're going to make it work, give it some air.