In article , Bill wrote:
[...]

3/4" sounded "right" to me to. I was planning to drill a practice hole
before I committed myself to see how well the cables fit! : )

3/4" is overkill for most NM cabling. 1/2" is ample for one or two 14-2 or one
14-3; 5/8" quite sufficient for one or two 12-2 or one 12-3. Why bother
drilling smaller holes, you may ask? It's faster. And it doesn't drain the
battery in your cordless drill as quickly.
[...]
Three 10-2 Romex cables in one hole okay?

As long as the hole is big enough to put the cables through it without kinking
or damaging them, sure. AFAIK, the Code doesn't mention anything at all about
minimum hole size through wood framing.

Attic. As the cable has to go perpendicular to the floor joists and many
of them are already covered with flooring, I latched onto the idea of a
"running board" when Mike Marlow first mentioned it. If I understand
what you wrote concerning where to locate it, is across the bottom of
the rafters, a few inches above the flooring, part of the "bottom truss
web"?

It can be wherever you like: near the floor, near the ceiling, or anywhere in
between, including run diagonally or in a logarithmic curve. It doesn't
matter.

The roof does not leak but should the proximity to the elements
via the roofing nails be of concern?

No.

It is good that no feet will be able to stretch the wire, but I think it
should be protected so that no one can push a box against it as well.

Correct.

I have seen the idea of building a "U-shaped" running board proposed.

Not necessary. Why make more work for yourself?

6 cables across an 6"-8" wide board?

No problem. A 1x6 should be fine.

If the location I identified above
is okay, I would consider nailing the wires with the plastic backed
staples to the running board with great care not to damage any wires,
and then "screwing" the running board to the rafters in case it may ever
need to be modified.

Three comments about that plan:
1) Wrong order. Attach the running board to the rafters first, then the cables
to the board. First, that way you don't risk damaging the cables as you attach
the board; second, I don't think you've considered the difficulty of attaching
the boards with half a dozen cables hanging off of them....
2) No real point in using screws. If it ever "needs to be modified", just
install a new running board and leave the old one in place. *By far* the
hardest part of modifying it later will be moving the cables. Whatever you do
with the running board at that time pales into insignificance by comparison.
3) Why bother with the plastic-backed staples? Code permits plain metal
staples such as these
http://www.amazon.com/NM-Cable-Stapl.../dp/B002CM2IPY
which are much cheaper, and *far* easier to install.