Any long cables with RF clocks/data will radiate noise and when connected to AC ground could also radiate via AC cable from path ground loops.
Smart idea is make generous use of common mode chokes for interface cables and DC outputs to raise impedance of the common mode signals from AC ground.
Beware that Hipot testing of AC/DC PSU may cause failure on secondary port when grounded. This is due to dielectric breakdown of Hipot from primary to secondary. Floating secondary is easier to protect if there is capacitive leakage and tester uses fast rising input. In this case modify Hipot tester to avoid discharge surge that would destruct PSU by using a current limiting series resistor.
Leakage can be minimized with prudent gaps, quality caps, and moisture protection on conductors. (moist dust causes leakage)
Your assumption in the second bullet point is correct: that answer does not apply to your situation. The linked question is discussing separate but electrically equal analog and digital ground planes, which is a technique used to reduce electrical noise, not to provide galvanic isolation.
To answer your more general question, you should either use multiple different symbols (you may be able to rename the net for the 'gnd' symbol in Eagle to create different nets; it's a while since I used Eagle), or use named nets for your separate grounds.
Your schematic as shown won't work as intended, however. The isolated side of your optoisolators is powered from a power supply that's (presumably) referenced to the ground of the Arduino side of the circuit.
If you want each of the four circuits to be isolated from each other as well as from the Arduino, you will need a separate power supply for each, and four grounds (plus the Arduino's ground). If you only want to have isolation between the CCFL and Arduino sections of the circuit, you will need a single isolated power supply for the CCFL side - and a single ground in addition to the Arduino's ground.
In every case, remember that you can only use power supplies and signals referenced to the same ground as that part of your circuit.
Best Answer
It depends on final equipment ground policy which may differ. In all systems i designed i use GND star connection, which means the following:
*there are sometimes grounds that somehow depend on other systems and cannot be connected. Most common: minus of diode bridge in AC rectifier. Another possibility- minus of isolated supply for current sensing, which is connected to a motor phase. But since you are not asking about it, we will not discuss it.
Also on each board near each mounting hole there are capacitors that connect the chassis to grounds in AC domain. This is important, because being thick (in relation to PCB copper planes) the case usually has very low inductance and resistance and may be considered perfect ground for noises of all kinds. But on the other hands you don't want any power to pass through the case, so the connection is by caps.
Speaking of ground policy, nice to see that you use optocouplers. Hope you did not forget power filters.