It ain't broke, don't fix it. The way your camera is set up, as long as you only press the AF-on button when you want to focus, you'll be fine. If you hold down the AF-on while you recompose, you would get out of focus shots. I doubt you're doing that, though, since the results are brutally obvious.
_____ Brian... a bicoastal Nikonian and Team Member
My gallery is online. Comments and critique welcomed any time!