In a perfect world we would have long timber and no need of scarf joints. I have never had to use a scarf in a tension situation. I also have never built any large bridges. Sills and top plates are where I find them used. I try to keep things simple and I don't hold back with a scarf, I keep it as simple as required, I won't put extra work into something that does not need it. My favorite scarf is the halved and bridled. If I need some extra grunt I may put a table in it and some extra pegs, even some iron.

Also in this imperfect world scarfs do not always fall in the positions Jim described. It would not be the end of the world if it fell on a post. I would not favor it out in the middle of the bay, though. I like Jim's example, it shows things do not always fall as neat as we wish.

An obvious item to be pointed out, cut the scarfs in the butts, the wood is usually clearer, knot free-ish, and square edged, as opposed to the tops which usually are bark plenty and knotty. Cut the scarf first, assemble it and bore any peg holes dead last, after all fitting is completed. Then assemble it fully and pull all layout including the docked ends as one long stick.