Depends on the situation. Theres alot of different ways to mic an acoustic guitar.
If i want it bright and need it mono. I will use a Soundelux U195 or something facing the 12th fret. Usually about a foot to a foot and a half away. But it depends on what kind of body i need in it as well.
If i want a very dry, rich, natural sounding guitar and its the guitar singular i will use 2 AT4050s. One at the neck angled a bit inward until it looses the high end crisp. Then the other on the opposite side doing the some thing but more on the butt of the guitar. That always gives me a nice tight and warm sound and blended together you get a nice wide rich guitar sound. I do this only when i need it in stereo with the vocals inside.
Sometimes i will use the MXL603s or AT4033s in place of the At4050s or Soundelux U195 if it seems to have a better sound.
Danny