@evansd2 is totally right if you’re interested in the well reasoned, conservative approach.
In this case I’m more of the “it’s just a phone case, they’re cheap” kind of guy. Most phone cases with veneer are glued to a plastic case anyway so the case will still be good even if you go through the veneer. In short though, it’s just a phone case, not an irreplaceable heirloom.
With wood veneer I’d let her rip with fasted speed, and probably 50-60 power for an engrave. Keep your LPI in the lower range (195-225) and don’t even think about scoring anything. Make a couple passes if needed.
This all depends on your image and how much risk you want to take.
You might also want to consider masking so you don’t have to try sanding and scorch marks on the finished surface. You don’t know how this wood veneer is going to react. 
One more thing, so that you can position your design just so. Make a jig out of cardboard. Make your first cut to hold the case in place and ignore your design/logo bits. Then insert your case and do your engraving after setting up your job again. Get the right height in for the case. Don’t forget to turn off the cut. Use pegs or magnets to hold your jig in place.
Good luck with whatever you decide 