I just had mine laser engraved yesterday (see here:
https://forums.macrumors.com/threads/1366494/). I called Apple support and the rep confirmed over the phone that it does not void warranty unless there is evident 'burn through'. Everything worked out fine and you can see my finished product at the link.
As far as advice, I would advise that when you have your artwork together and submit, be sure to request a proof and a pic of a test burn. Both of those can very greatly from your original visualization of what you are wanting done.
Lastly, if you engage someone to start working on your project and at the last minute, get cold feet, be prepared to at least pay the minimum artwork charge. In the long run, even that would be cheaper than following through on something that might keep you up at night, having tatoo'd your $1000+ machine.