That's not correct. If you think you might land with less than Fixed Reserve you declare “minimum fuel”. So in the case that you are given an approach time that would infringe your fixed reserve you either divert, if not committed to the destination, or announce “minimum fuel”, I would also suggest you mention the latest approach time. If during the approach it becomes known that extra delays/track miles etc have caused you now to be landing with less than minimum fuel, you announce the Mayday. In this case, on landing you would then need to file a safety report, and would probably be visited by the local authority to see what fuel you landed with for their reports. If enroute and you find you are going to land at destination with less than final reserve, you stop somewhere closer where it's still intact and refuel.
I would suggest for commercial ops A robust fuel planning section in the OPS manual would help prevent this. For private ops some pilot CDF as mentioned earlier would keep you from the doo doo.