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.