it kinda depends ...
On some masthead rigs, it's common (and even fast on some boats) to have a slack leeward cap shroud when beating in a good breeze. Often this is because the top section of the mast (above the top spreaders) is bending to leeward, which can help depower the main. Make sure this is not happening because you have too much back stay tension, or you are probably compressing your mast! Also, check to see that the slack leewqrd shrouds are not the result of having lower shrouds that are too loose, which allows the middle of the mast to bend to windward. However, if your mast is straight but is leaning to leeward on both tacks, both of your cap shrouds need to be tightened (and probably your lowers too). As a benchmark, perhaps you can find a similar boat (especially one that races fairly competitively) and get stay tension measurements from it (with a Loos Gauge designed for the actual wire/rod diameters present). My quirky french boat uses a wide range of rig wire diameters, which required me to buy two gauges (at about $60 each - ouch!) to cover all the measurements I had to make. If you have a fractional rig instead of a masthead rig, I don't think you should have loose leeward cap shrouds on the wind in a breeze. Under these conditions, the top of the mast (above the forestay attachment point) is free to bend to leeward in the puffs to depower the main. On these rigs, applying more backstay does not compress the mast or cause the top of it to bend to leeward - it just bends its middle section forward to flatten the main. Good luck!
