Now that's interesting! I have always used them level and not noticed any issues. Which manufacturer's instructions tell you to rest the guide on the depth gauge?
From a STIHL manual.
I’ve seen it other places too. Not sure if I 100% agree, especially, if you consider @Okie ‘s comments, above.
But, generally, I have seen instructions to only file level with it: NOT at a 10° ‘down angle’. STIHL does not recommend a ‘down angle’ at all.
Philbert