On my current project we had a requirement where based on the user browsing the SP Site, I needed to set the value of a MultiChoice Field to yes/no.

At first, I had difficult time figuring out why my code was not updating the value of my field.  Here is what works!  Specifically, the SystemUpdate(false) is what did it.

//always set the locked value to YES.
  properties.AfterProperties["Locked_x0020_and_x0020_ready_x00"] = "Yes";

  try
  {
      DisableEventFiring();
      properties.ListItem.SystemUpdate(false);
      EnableEventFiring();

  }
  catch (SPException ex) {}

Hope this helps you,
O