No, it's not cheating (I defy anybody to disagree) if you don't *** it in the wrong context. The mistake made most of the time isn't so much when to depress it, but how to depress it. Remember your brain now has to control 3 different movements, your eyes, your hands, and both feet. So you can't expect to master its use in a single practice session. It would help a great deal if you played a few parts of your previous repertoire pieces with 3 levels of the una corda. 1. the full depression, 2. half depression, 3. 1/3 depression. Each variation matters, especially in pieces that have long descending or ascending runs with the diminuendo marking.Good luck!
I think I'm going to have a Bernhard moment here: