It really depends on the servos. Digital ones will normally return to center. Analog won't. But it really doesn't make any difference**. Take the props off and arm it and start it so the servo is active and the motors are running. Then holding the copter in your hands, yaw it back it forth. You should see the tail servo correcting for yaw. If it does, chances are it's working.
Depending on which way you got your tail prop turning, and whether or not you are using counter-rotating props on the wing, the actual trim for zero yaw will be off to one side and not in the center.
**Steve, I would like to clarify what I mean when I say "doesn't make any difference". If the tail is out of hover trim on liftoff on your Tri, and the yaw PID's are set right, the gyros will determine that yaw has started on liftoff and immediately compensate for it, and steer the tail to the correct position to stop it. You may see just a tiny yaw on initial liftoff, but then it will "lock the tail".
I feel it's best to trim the tail with the mechanical linkage so it is in the correct torque compensation position for hover with the servo centered at 1500ppm. But that's not absolutely necessary since it takes more servo travel for yaw one direction than the other anyway. One direction you have torque helping to yaw. The other direction is anti-torque. Normally, the tail rotor will be tilted slightly left (looking from behind) in hover using standard tail rotation with counter-rotating wing props.