MOSFET in cadence is symmetric? Source and drain don't really matter diagramatically
Does it really matter which terminal is used for source and drain?


Yes.

The actual MOSFET is symmetrical operationally.

ie the channel is a voltage controlled two-quadrant resistor that conducts with either D-S polarity as long as appropriate Vgs is applied. 

HOWEVER, in almost all practical devices* there is an 'intrinsic" Drain-Source diode biased so as to NOT conduct when the FET is used with the normal polarity.
If you swap frain and source terminals you can still turn on / enhance an NMOS device by applying a positive voltage on the gate relative to the source.
You CANNOT enhance the device by applying a voltage to gate relative to drain.

*Integrated circuits using a non conductive substrate such as Saphire on Silicon do not have intrinsic body diodes.  You MAY be able to buy discrete devices where the substrate and source are brought out separately but in about 100% of real world devices the source and substrate are electrically connected internally. Having the source and substrate not connected complicates control of the device.