... a minimum fuel plan is unlikely to result in using FIXED reserve.
If 5% VR is all you have loaded, then all it takes is a slightly slower groundspeed than you planned for, coincident with a slightly higher fuel flow (say due to colder temperatures aloft), and you've done your VR. If you departed with minimum fuel, you will soon be into your fixed reserve and making that mayday call.
... you don’t suddenly discover that you’ve eaten all of your variable after you get to your destination ...
The main thing that people don't understand about VR is that, due to slight inaccuracies in fuel flow displays and fuel contents gauges, you may not know that you are burning it.