At full throttle your plane should climb. You should trim for straight and level flight at about 1/2 throttle. You should be able to control a climb or decent with throttle alone. A basic flight principal is that the primary control of altitude is by throttle and the primary control of speed is pitch. They are closely related and in RC are often interchanged. Think of the throttle almost like a trim. It is properly set for neutral at 1/2 throttle. A properly ballanced and powered plane should fly straight and level when all surfaces, including throttle are in thier neutral/midpoint settings.

Scott