I was not aware of such a rule of thumb. And certainly I haven’t seen it applied much in audio designs. Kassutronics Slope for instance:
Depends on the LEDs; most of the ones I’ve used have leads just long enough.
Some don’t, which is what prompted me recently to go looking for (and find) a new source of diffuse bright LEDs.
A question I’d raise is… How will you use this design? One thing is if using a single jacks board on a module you’d be limited to four jacks in a 10 cm module or two in a 5 cm module, and for most designs I think that’s too few. But you can’t use two boards having all four jacks on each on one module. Well, you could if you put one row of jacks across the bottom of the panel and one across the top. Or one up one side and one up the other. I definitely don’t like the former layout and would not want to use the latter much.
I guess you could use two jacks boards in two rows across the bottom provided the upper row has fewer than four jacks on it, and in particular one of the middle positions is empty. Then the main PCB could have a tongue extending through that position at the top jacks board to the header on the bottom jacks board. For a 5 cm module, though, you could not use a similar idea.
But I’d think an eight jacks version with two rows would be more useful. Of course if you do need only four jacks in 10 cm that would take up more room than needed, but a 10x20 cm panel usually gives you room to spare anyway.