Most pumps will have the inlet entering the center of the housing from the side with the outlet exiting from the top. If you have one of these, it's always best to have the outlet pointing up. If it's not pointing up it can lead to air being trapped inside the pump which will decrease performance and increase noise.

Its best to have your pump located at the bottom of your loop from a bleeding-the-air-out point-of-view as air naturally rises to the top.