I am not assuming anything. To date, I have not had any of my customers come to me and say, "after you engraved something on my device, I needed to take it to get service from Apple or an Apple authorized warranty service center, and was denied coverage because my device was engraved."
The laser I use cannot penitrate metal (N.B. Laser engraving is a thermal process. It could, therefore, hypothetically, generate enough heat to damage an electronic device by melting the surface or things just beneath the surface. However, to do so would take a very long time using a 30-Watt laser like mine, and a lot of conscious effort. It would be like intentionally putting the device in a forge. I'm trained and experienced enough to avoid creating conditions that would wreck my customers' electronics. I have to be non-foolish. I guarentee my work.) Also, unlike traditional engraving machines, lasers cannot generate any static electricity, which could potentially harm electronics. Therefore, custom laser engraving is very cool, permanent, and not-harmful to electronic devices, if you take it to an experienced engraver who isn't out to get you. 🙂