Some acoustic guitarists do not like too much tizz on the top or scoop out of the middle - they would rather have the wood. I know of one biggish name (I never did his PA) who told me he asks for a line feed and puts his own compact mic and equalizer on stage so that he can send what he thinks a guitar sounds like.

If a crucial guitar is pluggable it is a good plan at sound check to give the guitarist a really long lead (or a wireless pack) and ask him to come off stage to listen to the front of house. When he is happy you know what he likes, and you can re-tailor the sound as the audience comes in (audiences are a nuisance, they affect the acoustics of the room!) to try to keep the preferred sound.