They have a sense of humor, it's just usually rather dry... more based on subtle irony and silliness than the more brash and brazen American humor. In this case however, Jules is correct [on a technical level] about covering the first couple of unused holes on the patchbay.
While the unit is passive there is some airborne radiated high voltage transfer that occurs when the first couple of channels remain unassigned in a passive summing application. The phenomenon was first discussed in a white paper by David R. Berchard [AES, NAB] in 1947. It has been known as the Berchard Effect ever since. While rare, there have been several serious injuries and I believe one death that has resulted from the Berchard Effect.
There is a flip side in that if you do cover the holes on your jackfield for the unused earlier channels, and don't assign those channels to the mix buss, you will find a .018db increase in signal to noise performance per two channels not assigned. This benefit is also stated in the Berchard paper.
All the channels are identical, it doesn't matter which ones you use. The one thing that is important is that you shut off (un-assign) any channels that don't have something plugged into them. By "something" I mean an audio source, not just a patchbay. The passive summing network depends on a predictably low-impedance source on all channels, such as a line-level output on some other piece of gear. When you unassign a Folcrom channel, the source is disconnected and this low impedance is provided internally.