5v vs 12v dc pcb mounted relay selection
... that current dissipation in 5 volt would be higher.


Current isn't "dissipated" - it flows through the relay coils. The power is dissipated in the coils according to \$ P = VI \$. Since the power required to pull in the relay is 140 mW (a constant, whatever coil voltage is selected) then if the voltage is reduced the current must go up.

If 12 V is available I would tend to power the relays from that just to keep the switching noise away from the 5 V or 3.3 V logic and to reduce the load, size and heat on the 5 V regulator.


  Since circuit will be running 24/7,I intend to keep heat dissipation as low as possible.


The power dissipated in the relay coils will be 140 mW either way. The power dissipated in the switching transistor, however, will be proportional to current so the 12 V relay will be lower. 

Your decision will probably be based on available supply voltages.