TubeNet

No. Normally when no valves are depressed the air stream does not go through any of the valve tubing loops. You can easily test that statement by removing the valve slides. (But keep the main tuning slide.)
You will then be able to play all of the open bugle notes, but pressing any valve will result in a "hosepipe" sound because you are no longer sending sound waves into the bell.

ExaltedCashews wrote:So, when building a horn, must you consider the length of the valve branches in the length of the horn? Or, is it the length of the horn minus the valve branches?

To set the fundamental pitch of a horn you only have to consider the length of tubing through the open bugle.... From the mouthpiece to the end of the bell... including the pathways through the pistons or rotors.... but NOT through the individual valve circuits.