Hi,
There are two important considerations,when recording acoustic instruments:
1.Is the instrument in question going to be the FOCAL instrument (i.e. Bob Dylan Acoustic song?)
2.Is the instrument in question going to be a background instrument,or part of a larger ensemble (i.e. rhythm guitar on a rock tune.)
IF 1, THEN you will benefit from recording the instrument in a STEREO configuration,and make sure you get some of the room ambiance as well,on a third track,if possible.There are many ways to record this way,I suggest you research the BLUMLEIN configurations.There are several to choose from,depending on what microphones you have available.The goal here is to make the instrument as large sounding as possible,like a concert piano.
IF 2, THEN you want to record the instrument in a MONO configuration,and make sure NOT to get too much of the room ambiance,as this will be counter to the further processing you will do later on to marry the instrument with the rest of the ensemble.In this caase,you want to choose a microphone that will accentuate the frequencies you will want to hear in the mix,and reject or minimise the ones you do not want in the mix,to make it easier to blend the instrument with your ensemble.Pick a smaller pencil condenser lie a Neumann KM84, or similar,and roll off as much as 120Hz.
Cheers