Actually Tabndust is right, if you have room put valves at the input and output so you can isolate the pump and remove it for maintenance without having to empty the pipes. Use ball union valves with the union to the pipe side of the pump.
Now having said that you can only use the valve in the pump ouput for flow control. The reason has to do with the design of the pump. Centrifugal pumps require net positive suction pressure in the inlet to function properly, if you restric the flow at the inlet the pump will try to pull instead of pushing creating negative pressure (vaccum) at the pump inlet, this vaccum generate bubbles (called cavitation) which increase noise and vibration with potential damage to seals and/or bearings. Also cavitation will reduce the pumps performance wasting power.
You do not have this effect if restricting the pump outlet.
By the way, for improved performance try selecting a pump that closely matches your flow requirments and if possible increase the diameter of the inlet. Do not guide yourself by the size of the pump connection at the inlet. Most Iwakis have 1" inlet but require 11/2" or 2" in the inlet. Mag drives 9 and higher have inlets of 3/4" but the manufacturer recomends 11/2" for the inlet and discharge pipes. etc.
