I was taught that in light air and off wind it's desirable to have more sag, and when beating to windward less sag lets you point higher.

I have adjustable backstays and can't ever get the headstay straight-bar tight. My boat's maker says not to use two hands on the winch that adjusts the backstays, and always adjust only the windward stay (never the leeward).

Unless you're a competitive racer, my intuition tells me it's not good to stress the rig to its limits.