I don't know the routine at most places, but where I worked, someone clocked in, came over to you at your position and relieved you, you'd tell them about any problems that had happened during your shift, you went over to the time clock, and clocked out. I remember the one time I actually clocked out a couple of minutes early and got a formal reprimand (even though the only thing I would have been doing during those two or three minutes would have been sitting on my butt.)

If you're in a union job, you follow their rules. Period. No room for leeway. That's one problem with most unions - they can apply power to their company(s), in terms of negotiating contracts with the employers, but they also apply coercive power to their members, and end up with lowest common denominator one-size-fits-all rules.