There has been a very long discussion of steward before on these forums but it mainly centered on whether to play the steward for 6 coin hand on turn 3. Opening steward/steward was mentioned as a radical new idea, which shows how long ago that discussion was. Back then I suggested that a steward was at its best when you could use all three functions and whilst that's generally true it is of no great value as advice. Nobody should be trying to invent uses for their steward when there's one good thing the steward should do all game long. Generally, however, the steward is a decision card and there isn't a good article about steward since nobody can give specific advice that matches every decision players will face.
-Trashing uses 3 cards from a 5 card hand and hurts income. There needs to be some benefit later.
-Two cost cards are significant when using stewards since you are likely to have hands where you trash and have 2 coins to spend.
-Villages can be important for steward, particularly villages with +cards.
-The throne room family works very well with steward, even allowing decks entirely full of terminal actions.
-As a note for new owners of the intrigue set, steward defends well against torturer.