It should not be tightened down as it will just introduce unnecessary force and add more friction. It just needs to hold the ground board on so it doesn't fall of. What matters more is ensuring the lateral movement has no slop so that it turns rather than shifts.
I also strongly recommend a teflon bearing or stack of milkjugs in the center to reduce the load carried by the bearings on the periphery. Makes a *huge* difference in how smooth the azimuth motion is. Just have to find the right thickness so that the dob doesn't want wobble.
The center bolt should not exert a downward force.
The difficulty with shimming the center bearing is that Dobs are wood and nothing is perfectly flat. Avoiding the wobble at high mags and yet actually having the center bearing carry some load is not easily done.
I don't worry much about friction, I worry about stiction. Stiction is what causes a jerky motion. I don't worry too much about balancing the altitude and azimuth force, the altitude force is more or less constant but the length of the azimuth lever depends on the altitude, near the horizon, the lever is long, the force low, near the zenith, the lever is short, the force high.
Since I began using soap to lube the bearings, stiction has not been a problem and friction has been reduced somewhat.
For my 16 inch, I designed and built a spring loaded center bolt. The spring pushes up on the rocker box and provides a constant force even if the ground board and/or the rocker box are not perfectly flat. The force is adjustable and can be adjusted with the scope assembled. It works.
The center bolt is secured by nuts top and bottom to the ground board so it's rigidly attached. The actual bearing is in the top plate.