Quote Originally Posted by tot_Ou_tard
1) Always make the change: it does wonders.
2) Do it if you have extra cash: it'll help but it's not necessary.
3) It's a preference thing: It has it's advantages & disadvantages.
4) It depends on the composition of the current nut or saddle.
5) Only do it if you are having issues.
6) Don't do it. It's expensive & doesn't give you anything.
7) Bone sucks.
I'd point out that changing the saddle will be free fost most guitars; that is, it will cost whatever the new saddle costs. Current production Martin guitars, for example, have a drop-in saddle that you can just pull out with your fingers or some pliers with a rag wrapped around the sharp bits - then you just drop the new one in, and adjust it by removing whatever you need from the bottom with sandpaper on a flat surface, then re-stringing the guitar and check the new height.

In my humble experience, it almost never pays off to replace the nut if it's working right, but it always pays off to replace the saddle with Tusq or bone if it's something else. Corian is also OK for most guitars (Martin used to use Corian a lot, and Gibson keeps using it for nuts). It makes sense if you think about it: a major open A chord uses 5 strings, of which 3 strings go over the 2nd fret, not the nut. The nut never comes into the equation for anything other than open chords, and even then it tends to influence only at most half the strings in a chord, if it influences much anything at all.

But the saddle always comes into the equation. I've also found, reading William Cumpiano's fine book on acoustic guitars, that the saddle height (and in part, the material the saddle's made of) has a big influence on how loud the guitar is: a higher saddle means that the volume knob is turned up, with a lower saddle you turn it down.

So - don't replace the nut unless it's broken or you can have it done for free; replace the saddle if it's too low, and not made of either bone or Tusq - and think about it twice if it's Corian and you don't hate the sound of your guitar. I believe that with guitars, just as with people, unnecessary surgery should be avoided.