What BenQ are you using?
The home theater line of BenQs (PE7700, 8700, and 8720), have a grid pattern that you can access from the user menu. Displaying the grid pattern can help in leveling the projector.
The BenQs also have an offset of a few inches at app. 13-14 ft. (see the manual for the exact offset based on the projectors distance from the screen). This means the top of the screen will be a few inches below the middle of the BenQ lens.
If your screen is level, your BenQ is at the proper height and distance, and the BenQ is level, your image should be level unless you have the very unlikely problem of the DLP chip being skewed.