In regards to modal double-faced cards there are many effects which allow you to "Cheat" in one side or another.
If an effect allows you to play a land or cast a spell from among a group of cards, you may play or cast a modal double-faced card with any face that fits the criteria of that effect.
However does this effect apply to targeting abilities? If one has a Birgi their graveyard (Birgi, God of Storytelling / Harnfel, Horn of Bounty). Her flip-side is an artifact. Would one be able to Sacrifice an artifact (a Myr token lets say), with something like Goblin Welder or Daretti, Scrap Savant 's -2 to reanimate either side of Birgi?