It's always fun too record the acoustic guitar and as trippy states, " a little movement goes a long way". There are so many different approaches because every ear and genre likes something a little different. For bluegrass, I think a ribbon(the fat head sounds nice and won't break the bank) about 12" away from the 12th fret and an Omni 3-4 feet away. High pass the ribbon and add some upper mids and highs. Add a little verb to the close mic and blend the two mics to your liking.
Again you just have to find what you like. Sometimes a 57 will do the job with a little compression and the right EQ. It all sounds good, it's in the ear of the beholder.
