Given that the marks would be covered, it would be cool to preserve them as original. I agree that most people would take the photo and reproduce them, but that's reason enough right there to not do the same.
Mark - thanks for posting your picture up. I've had the entire interior out of my '69 convertible and there are not any original crayon marks inside.
Triton2110 - I specifically bought my '69 knowing that it wasn't a numbers matching car so that I could make modifications to it. You haven't said whether you intend to restore it to sell it, or to keep it. If you're going to keep it and drive it, you might consider upgrading to discs, adding HEI and/or EFI and upgrading in the suspension areas to make it drive much better. If you plan to sell it, clone it to SS or RS/SS standards. Please just be very clear with the buyer that it's a clone.
Either way, a '69 Camaro is a stupid simple car to work on. I've learned a ton over the years, and I still have big plans for mine. Enjoy your car!