Parallel networks and references
You can (in general) select the polarity of current sources and voltage sources any way you want.  Then, when you solve the circuit (via nodal or mesh analysis) if you guessed wrong on any of the sources the voltage or current value will come out negative.  This tells you the actual direction is opposite from what you picked.

Good practice is to choose initial polarities to be consistent (as you suggested) and then solve the circuit!