OK, I have moved the "do not set to zero" logic to the service callback.  I had completely spaced on wheel mode and I now see you have enableWheelMode and disableWheelMode functions in arbotix.py.  I'm thinking if someone wants a service for setting speed in wheel mode in the future they/we can use "set_wheel_speed" for the service name.

The default goal speed for an AX12 appears to be 0 even after setting a new goal speed and then power-cycling the servo.  I suggest a value of 100 (from the range 0-1023) which seems like a moderate value without being crazy fast like 0 is, but I leave to you if we want to set the default at start up which would break backward expectations.