I jet mine up and out at a 45deg angle above the water surface, this forces a great deal of oxygen into the water (and makes a very tranquil splashing sound covering the sound of the pump and filter) - it depends on how active/peacefull you need your water surface to be I guess. The instructions on the box indicated that it doesn't matter what angle you have the flow, only that different angles give different surface conditions/currents.
I discovered something fun with mine last night. If you take the spray bar off and use some of the other pipes that come in the box to aim the flow straight up in the air, an xp2 will give you a fountain 3-4 inches high that looks and sounds great and gives you enough surface disturbance to create bubbles, but not so much that fish are afraid to come up for food. I love it! My own little tank fountain.