Our procedure (747 Classic) is to set MAP at the Final Approach Fix. After that, we are using Glideslope or Vertical Speed for vertical guidance, so Altitude Capture is not a problem and setting an altitude other than the "target" altitude will not affect Flight Director guidance.
If you are at level at 3000' intercepting the localizer, cleared for the approach, and Altitude Hold (or your equivalent) is engaged, the MAP could be preset after ILS mode is selected and prior to G/S capture.