The reply about cropping a color matte can be taken a step further.
Either a matte in FCP, or a grid created in FCP- place this clip on the top timeline track in FCP. Lock the track. This way, when you click into the Canvas to re-position your image, you won't accidentally select your matte or grid, as its locked in place, just the images on unlocked tracks.
Good luck,
Andrew Balis
Lead Final Cut Pro Instructor
www.moviola.com