how much low is low?
Since low is needed because of capacitive coupling, low depend on PCB layout, selected chips (a 100MHz cygnal will, regardless of clock speed, couple harder than an old sloven 8051. The time to calculate "how low" for a given PCB will be more than anyone want to tackle.

I do not make single or doubl sided PCBs any more, the added cost for being safe is neglible.

Erik