Is it good practice to couple the grounds of isolated supplies with capacitors
Probably best answer is "depending on application". Sometimes grounds are not necessarily "silent" node (for example, powe ground in motor driver), then it should not be coupled with anything else. On the other hand, digital and analog grounds are supposed to be clean, and for that i usually couple them to metal enclosure, which is a thick (related to pcb planes) piece of metal with very low impedance, hence with equal voltage across the whole thing. And of course, the grounds are then coupled between themselves.