I found some interesting text on www.alliedluthier.com;
http://www.alliedlutherie.com/truss_rods.htm

Quote :
Truss rod installation:
These truss rods should be installed with a filler strip over them. Installing a filler greatly reduces the possibility of a truss rod rattle and reduces the amount of backlash in the rod. My installation procedure goes like this: Mill a 1/4" wide, flat bottom slot in the neck that is at least .400" deep. Make a filler strip approximately 3/16" thick that snuggly fits the truss rod slot. Rough up the flat backbone of the rod with very course sand paper or a file. This creates a good gluing surface. Put the rod in the slot with no glue. Apply epoxy to three sides of the filler and press it into place over the rod. Don't overdo it with the glue. You don't want it to squeeze down and foul up the threads.
Apply LIGHT clamping pressure along the length of the rod (see photo). You want enough pressure to eliminate any space between the flat bar and the round bar, but not so much that it makes the rod hard to turn. Check the resistance by turning the rod a quarter turn in each direction. You will notice that the filler strip sink in a little more in the middle than at the ends. After the glue dries, level off the protruding filler strip. Avoid turning the truss rod more than 1/2 turn until after the fingerboard is glued on and fully dry.

Is it really necessary to use a filler strip? Isn't it enough to make a really snug fit for the truss rod and glue the fingerboard on it?

:hmm:

I thought that (a) the channel had to have a bow in it, and (b) the truss rod fillet strip then copied that bow, holding the truss rod in a gentle curve so it could do it's thing better. Without the fillet the rod is not held in the correct passive curve shape.

Is it really necessary to use a filler strip? Isn't it enough to make a really snug fit for the truss rod and glue the fingerboard on it?

Well on the first part, yes you need the filler, its what the rod pushes against to straighten the neck. The bow thats in the rod will want to straighten out as you tighten it, and it turn it pushes up on the filler. Second the rod really shouldn't be in the channel that tight, it needs to be free to move. And for whether the channel should be concave or rectangular I think this is more of a structural consideration than anything else. Concave will leave slightly more material in the ends of the channel than a rectangular cut would, and in turn make it slightly stronger.

The OP is talking about a double rod, which goes in a straight channel with a rounded or squared bottom. Al Heeley is talking about a single rod, which requires a channel that curves along its length (along with a filler strip that does the same). Different animals.
And dman is talking about whether to use a round or square bottom channel on the straight slot for a double rod.

Pilotjones is right. Al and dman obviously thought that this was a regular (old style) kind of rod. but it isn't. This about these "new" 2-way truss rods, wWhen it's turned one way the rod turns one way. When it's turned the other direction the rod turns the other direction.
So the question still remains unanswered...
Lets put it this way; Is it common practice to install a thin strip between the rod and the fingerboard in the channel of the neck, EVEN if the truss rod is this "new" type of truss rod that can either make a frontbow or an backbow on the neck?