Hard to say. It depends on where the currents go.
I use copperclad ground plane for prototypes regardless of the frequency.
When you have devices that switch in sub-nanosecond speeds, wideband op amps, low level signals to track, anything to do with a PLL, etc, a solid ground plane is essential. I would rather spend my time improving a design than waste it on troubleshooting grounding problems.
I will also give here a secret I have been keeping for decades.
If you put a noisy circuit on its own copperclad, and use the minimum number of ground connections to the main pcb selected at optimum points, you can cut the crosstalk from the noisy circuit very significantly.
This is especially significant in low level circuits, PLL counters, VCOs, phase detectors, and anywhere else you are working with sensitive circuits.
Once you have the circuit working with minimum noise, you can easily transfer the layout to a working PCB.
This sounds similar to the daughterboards used on some oscilloscope front ends, although they seem to be using it to isolate the input signals instead of reducing crosstalk to or from other circuits. It is not clear how the daughterboard could reduce common mode switching noise from the CPU from going into the scope probe ground.
So the idea is to figure out what the currents are you have to deal with, and where they come from and where they go.