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It's interesting to look at the way a guitar top vibrates before you glue the braces on. It turns out that it does pretty much the same things a braced top does, but at lower frequencies. You could make the unbraced top thicker, and get the stiffness and the pitches up, but it would be pretty heavy. So this suggests that the function of the braces is to stiffen the top up without adding too much weight, and without necessarilly altering the way it 'wants' to vibrate. Of course, the top on the box is a different beast, but sauce for the goose....
My feeling is that the critical area to strenghten is between the soundhole and the bridge. a certain amount of 'belly' behind the bridge does not seem to hurt the tone, but most of the guitars I've seen that were 'played out' had a big dip in front of the bridge. You'll notice that the two most successful brace patterns, X-bracing and fan bracing, concentrate bracing in that area.
I feel that almost any bracing scheme will make a good sounding instrument in some sense so long as you get the bracing into some sort of 'balance' with the stiffness and mass of the top. Braces tend to be 'lumpy', adding a lot of stiffness or mass at certain points. If you think of the sound at high frequencies as spreading through the top sort of like ripples in a puddle, lumpy braces can act like rocks that stick up, or even just approach the surface level; reflecting those waves and breaking up the patterns. It's those resonant patterns that shape the sound, and killing them off with lumpy bracing seems to hurt the tone most of the time.
Of course, a guitar that sounds good to a Classical player when strung with nylon strings might sound awful in a Bluegrass group when set up with steel. Assuming the bridge stays on... Different kinds of music have different requirements, and you need to fine tune the bracing to work for the intended use, as well as working with the top it's glued to.
The 'standard' bracing paterns have been worked out to give the sort of sound people tend to like for the 'usual' sorts of music. The further you depart from these standards the greater the risk that the guitar won't sound 'right'. Deeper analysis can suggest reasons behind the choices: why X-bracing works well on large steel strings and not as well on small Classicals, for example.
In his talk at he last H'burg festival, Mark Blanchard said that 'the tone is in the top': all you can do with bracing is fine tune it. Of course, the differences between good guitars are pretty small, and that fine tuning adjustment matters. If he's right (and I think he is, or I would not have brought it up), then picking the 'right' top for a given shape/size/use is a key part of success. After that comes using the 'right' bracing pattern/profile to get the basic tone you want. After that comes the fine tuning to get that tone to be as good as it can be. It is distressingly easy to kill a good piece of wood with bad bracing, but getting it right is mostly a matter of paying attention to details and working carefully.
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