Op-amp variable reference voltage
Stability realy depends on the nature of the signal coming in and the offset that you are trying to adjust.

If the incoming offset you are trying to adjust out also tracks the supply rails you are using, this method works pretty well. If however the input signal has a fixed offset generated by some other circuit or device then a fixed reference would be more appropriate.

Basically, the answer is... it depends.

As for the capacitor, some filtering on offset trimmers is often a good idea, especially if the resistors involved are large. However you have to be aware that this capacitor introduces a settling time at power on which may or may not be important to your function.