Is the data in header message automatically filled?

edit

For the standard header template I'll need to us a line such as: poseName.header.stamp = ros::Time::now(); and poseName.header.frame_id = #; where # is some number of the associated frame?

You'll use whatever is correct. The pose you send out will represent some pose relative to some frame, that is the frame_id to set. Wherever that comes from will also determine the time to set. This is the time that pose had relative to this frame, not when some calculation was made.

frame_id and time stamp make sense in connection with tf. If you don't know how that works, check it out: http://wiki.ros.org/tf