Relay Connection 120V
There will be no difference, from an electrical standpoint, if you connect the source to the NO or to the COM terminal of the relay. That includes the fact that there will be no different behavior with regard to contact arcing. If the load that you are switching with the relay is one that is prone to causing a lot of arcing you may want to consider also using some additional components in the circuit to reduce this behavior. Such scheme would be called a snubber circuit should you wish to research the subject further. 

Since you do have SPDT relays there is a lot of common sense to the scheme to connect the source to the NO terminal. If you connect the power source to the COM terminal then there will also be the powered NC contact during the time relay coil is not energized.