I'd figure that for the most part, the load calculation is for upwind. As unless you are on something which speeds up a lot as you begin to bear off, apparent
wind falls as you move away from the
wind. And thus, so would the sail & sheet loads.
Although on a
displacement boat, the loads on the
spinnaker sheet, & especially the guy are HUGE when you're beam reaching. On that point of sail, the loads on everything are pretty much the highest which they get.
Now if you're talking about a lightweight
racer, or a multi, who's speed rises on a whole different curve as you foot off... even to the point where the
boat never sees wind aft of the beam, then that's another story entirely.