Rounding the Bottom of a Machine Screw will definitely help to hold a Small diameter wire in a Post Hole. But it can be a PITA sometimes to do without Hosing up the
Threads on the Screw.
Whenever you work on the End of a
Thread, here is a Little Trick.
Put a Nut or enough Nuts on the Screw
Thread to cover the Section of the Thread you Don't want Damaged. Then File Away. Or Chuck the Head of the Machine Screw in a Drill and "Mini Lathe" the end.
Then when your done, removing the Nuts will "Chase" the Thread. And Most Times knock off any burrs left from Filing/Sanding.
Working down on the M2 or M2.5 Level is a Drag. And I would Always Buying Something before Filling or Sanding. But if a Person has to, the Nut over the Thread Trick comes in Very Handy.