Do I Need to Keep the Base Station Plugged In When Not in Use?

Yes - if possible.

Although it is not absolutely necessary to keep the Base Station plugged in when not in use, it does assist in data logging on the back end, where the Base Station is actively connected to the internet.  

In addition, keeping the Base Station plugged in, and keeping the Smart Sock plugged into the Base Station when not in use, will ensure a complete charge when you're ready to use the Smart Sock, and troubleshooting any issues that may arise - like losing WiFi connectivity, can be addressed before you're needing to use the monitor for the night.