If you're using a rate gyro, you need to have the true center absolutely perfect on your linkage setup. When it's hovering, you need to either make linkage adjustments until it stays perfectly centered or slide your servo on the boom. Prepare for a LOT of time & frustration to get a rate gyro working well. Now TRUE center and MECHANICAL center are two different things. Mechanical center is when the servo's centered and the blades have no pitch left or right on it. True center is when the servo is centered and the tail blades have the right amount of pitch on them to counter the rotation of the blades. I wouldn't mess with your rudder trims until you get it close, it'll just add to your frustration. If you want, once you get it close, you can probably trim it to get it perfect. I hope this helped and I didn't over explain it and insult your intelligence. Good luck!

An afterthough, the little link ends on the tail slider that actuate the tail grips tend to have a spring action that makes the tail not center well. The linkage will feel smooth, but the center will seem to bind a little. You have to loosen the screws on the rear (inside) of the slider to let the eyelets not only turn, but actually move in and out as the tail grips rotate. I had this problem with mine. Make sure you check this before trying to set your tail up or it won't center well.