Don't be rude, there's nothing wrong with trying to keep your case in a good condition.
The appropriate advice here, however, is that leather cases aren't the best cases when it comes to longevity due to its material - at least if you wish to maintain the original appearance of the case.
Sorry, didn't mean to be rude, was just a bit surprised by the question of 'avoiding marks'.
Avoid carrying an iPhone alongside keys and loose change, with or without case. Leather is soft, glass can be scratched.Avoid carrying an iPhone in a leather case in a pocket alongside keys and loose change - leather is very soft.