On the flip side....here's another classic installers conundrum I've ran into more than once:
You have a long set of letters to install on a brick wall...
You level and hang your pattern...
You realize the baseline of the letters on the pattern, although level, isn't following the mortar line...
Although not obvious to the naked eye, the building/brick/mortar lines isn't level...
If you install the letters "correctly" so that they're level, they'll look off and stand out like a sore thumb because they won't follow the mortar line...
If you follow the mortar line - no one will ever notice it's not level, but it's technically "incorrect".
It's something that used to frazzle me, but I've learned to put the decision on the customer. Explain it isn't level, SHOW them it isn't level, and then ask them if they want it installed level or if they want it to follow the mortar lines. In THOSE cases, I encourage them to follow the mortar line.
No one is ever going to say "that mortar line doesn't look level". But they WILL say "those letters aren't level - look how they don't follow the mortar lines".