Except as a JSer, you are not a non-rev. You are officially a member of the crew, even if in back.

Maybe it’s different at different carriers, but at mine all JS are generally expected to check in.

Another pet peeve of mine is attire. My last line check, I had a FA show up out of compliance with company policy for attire. Put me in a super awkward position with the LCA because I didn’t notice the leggings, he did. Gee, thanks… Fortunately, she had proper clothes to change into quickly, and had a good attitude, so it was nbd. If you are going to exercise the privilege of using the JS, know and follow the rules for the company from which you are asking for a free ride. Both of these faux pas will almost certainly not get you kicked off my flight, but again, treat the privilege with the respect and courtesy it deserves.