Dennis,
The OPA228 family is meant for application with signal gains of 5 or greater but it has sufficient phase margin to maintain stability in unity gain with purely resistive loads. For applications that include the output capacitance, OPA228 is stable only in gains > 5 and placing any unused channels of OPA4228 in a unity gain may cause them to oscillate. Even though the oscillation itself will not damage any of the op amps, it may feed the unwanted signal through the shared power supply into the op amps that are being used and thus cause undesirable distortion.
The best way to prevent the oscillation is to place the unused op amps in gain of 5 or greater and to make sure that depending on the power supplies used the input common-mode and output voltages do not violate the linear ranges (see below).
As a side note, it is also possible to use OPA228 in lower gain than 5 and to remain stable, while driving some capacitive load, with the help of properly chosen feedback capacitor but you should be very careful about compensating the circuit as the PCB parasitics are not considered in application note below.
