I just started filing supervisor grievances myself. Volume is down so they started pushing our midnight start time back to the point of me losing on average an hour a week even doing extra work at the end of the night. Yet with volume down I still see supervisors working, so I started filing, though I do have a general rule that I'm following. Less than 5 minutes, it'll take me longer to write the grievance, but I'm willing to change that if this keeps up too much longer.
I've talked to my hub manager when he asked me about my grievances and I told him flat out, "Volumes down, we're getting our start time pushed back. So there's no reason for a supervisor to be working." He agrees, but I still think he won't do anything til I have a mountain of grievances or more people start filing, either way I'll get paid eventually.
Keep in mind everyone has a point where they can't ignore the problem anymore, after 5 and a half years this was mine. I work with a steward, and give another a drive into work, and they both agree that everyone has a point and that they can't force people to file. So my advice is to look for your reasoning and set yourself up a way to do it. No matter what, you're going to get some flak for it, just do your work and keep an eye out while never being afraid of calling them out on any retaliation attempts.
*edit - forgot a "they"