1. Obviously depends on the other cards. If there are $5 trashers you probably want those first. If there's a spammable $4 you probably want this early. With Swamp Shacks you want a lot of Quartermasters.
2. At some point you're going to switch to estates when you run out of actions that you want. I'm not very good at gauging when to do this.
3. Wall. Don't get Quartermasters when there's Wall. I did this once and was very sad.
I mean it's not that bad for wall as instead of gaining a card you could just keep Taking a card (assuming there arent any there)
They are really good in games where you get points for acquiring cards that you don't really want in your deck like Museum when there are a lot of stop actions, or Plateau Shepherds.
Generally speaking if you are going to get one, you probably want to get it early