When setting up your rigging, it might be very tempting to tighten each stay until something breaks! The problem with this protocol is you would then have something break.
The truth is, your standing rigging only needs to be tight enough to hold the mast straight and upright. The looser your standing rigging can be, the less stress you will put on all the fittings that are associated with the rigging. The removal of unnecessary static loads will greatly prolong the life of your rigging and your yacht.
It is important to remember what each stay does, and therefore, how tight it needs to be. The cap shrouds need to be the tightest of the shrouds, as they keep the tip of the mast in place. Due to leverage, this stay needs to withstand a lot of load. If the stay is slack at rest, the top the mast will move to leeward as the weather builds. Eventually, the stay will become tight and the mast will stop moving. The less you want the masthead to move, the tighter this stay needs to be.
The further down you go, the less stress each stay will experience, and the looser it can be.
On cutters, check stays will be present to counteract the inner forestay. These stays do not need to be tight at all, as all these stays do is stop the inner forestay from pumping the mast. As the inner forestay pulls forward, the mast will bend forward slightly, and the check stays will become tighter.
At rest, however, these stays may appear to be "too loose."
The headstay and backstay are another stay of constant debate. If they are tighter, you can point upwind better. Looser and you can reach better. The correct answer for the tension in these stays is "enough to point what you need." If you are a gaff rigged cutter, you will not point very well due to the sailplan, so there is no reason to overstress your fittings by having a bar-tight headstay. On the other hand, if you have a fin keeled racing sloop, the headstay should be akin to a banjo string!
Standing rigging is only there to hold the mast up. The goal is to accomplish this task with the loosest stays possible.