You have changed the subject a little bit. No one suggested NOT clocking out before going to lunch. The question was about which clock to use, and when.
When I come to work, my hands are clean. I should be able to wash them prior to clock out. After doing so, if they want me to walk past a perfectly good clock (near the restroom), to use the dock clock, fine. Not a problem.
Also no one cares what an office worker (or anyone else) does on their lunch break. MOST bring their lunch. They are certainly NOT expected to clock out AT THEIR DESK, are they?
The dirt on my hands is not Swamratt's dirt, it's FedEx'e dirt. The blood and sweat is all mine, not the dirt. Do you expect a mechanic to clock out with gobs of grease, grit and grime on his/her hands? Certainly NOT.
The rule enforcement suggested by the O/P is petty and unproductive. Many docks have more than one clock. One is generally close to the office. The intent is to be able to clock in/out in close proximity to the work area, not to prevent common sense efficiency, nor prevent washing hands on the clock.