We just use the ones that smell the best. Yesterday at Liturgy, I put some old incense on the coal after the good stuff had run out, and it gave a horribly bitter smell. I was talking to my priest afterwards, and he said that a young man told him once that it was to remind us of the "bitter-sweetness" of Christian life. My priest thought this was nuts; he said that incense is to glorify God, and God is not glorified "by stink."
I think just use the one that smells best; don't stink up the church, it's supposed to be a "sweet-smelling sacrifice."