So, knowing how each mic differs from the others helps to know how placement will change the sound.
Also, it's good to know where to point the mic at the instrument. At least a general idea, since these are guidlines and not rules. The basic founding pronciple of recording seems to be, "How does it sound?" The sound of the thing being recorded seems to be paramount, as it should. Furthermore, the desired sound is more important. What is the sound you want to achieve? Listen to it, try it out, and then mic it.
Last week, after going over mic types with my mentor, I watched another engineer use a spaced pair of mics to record an accoustic guitar session. It sounded great.
Doing it well...things are really getting fancy!