Should I be concerned about fridge/freezer airflow?
You should check the specifications for your fridge and freezer, specifically the section that discusses required clearances.  If you provided enough clearance, then the heat is likely not an issue as the fridge/freezer is designed to dissipate heat given the specified clearances.

If your clearances are too small then there is a good chance the device will not be able to properly dissipate the heat it produces.  This could possibily cause a fire, but the most likely scenario is that the devices will fail early due to components overheating.  

Adding ventilation is not necessarily going to help, certainly not from a warranty perspective, but ensuring the correct clearance will.

The other thing to consider is the impact of the heat on your cabinets.  Ventilation will help this.