I just want to add my opinion to ISOLATE EVERYTHING CONNECTED TO THE AC LINE!
There are few applications where I would not isolate from the AC powerline. Also, keep in mind that if you don't connect something like a 1M resistor from circuit common to the AC ground, you may have problems with static buildup.
With a low power circuit like this, probably not much capacitive coupling. I have seen higher power circuits with more capacitive coupling in the power supply transformer cause ungrounded isolated circuits to float at 1/2 of the powerline voltage, with enough coupling to cause a small shock. Those were arcade games with cheap replacement switching power supplies.