Where to ground low pass filter before dual supply operated op-amp?
The correct place to ground it is to 0 V. It is the reference for all voltages in your circuit.







simulate this circuit &ndash; Schematic created using CircuitLab

Figure 1. For perfectly regulated power rails C2 and C3 would perform in this application exactly the same as C1. 

Theoretically you could also connect the bottom of the capacitor to -5 V or even +5 V provided they are very stable. The capacitor in any of the three cases (+5, 0, -5 V) will provide an identical impedance to AC signals but will just have a difference in the DC voltage across it. 


The reason for using 0 V normally is that the power-rails usually have some noise on them due to variations in current demand and the voltage drop that this causes along the wiring / connections.
Another reason would be to prevent switch-on transients. If the C2 option was used then OA1 non-inverting input would be pulled to V+ on switch on. Typically we want to avoid this kind of action. In an audio circuit, for example, it would result in a large DC "thump" from the loud speakers.