I suppose a lot of people are trying out the cards online, but I only play IRL so all I can do is speculate. So here's my serious speculation about the strategy around boons.
Aside from Druid, you can't choose what boons to receive. So it doesn't seem like Fate cards play any sort of clear role. You want a trasher, best get a trasher instead of banking on a Fate card that is equally likely to gain you a silver. You want a gainer, best get a gainer instead of a card that might instead sift. If there's no other available gainer/trasher/buy, then you're SOL because the Fate card isn't likely to give you the one gift you want.
So when do you want boons? I'm guessing you want them early on, because most of the effects help you as you build. But it probably depends on the Fate card (e.g. you want an Idol whenever you want Silver+).
I think there are some Boons you will be happier to see than others. Flame, Swamp, and Earth seem like the best ones overall. I think you will rarely be happy to see Sky.