Even guitar amps deserve listening to as each speaker sounds slightly different. Yes, you can accomplish the same thing by having someone moving mics around from an eye driven position while you listen in the control room until your mic hits the sweet spot, but doesn’t it make sense to go find the sweet spots first?
Sometimes you may need to find different spots that emphasize different parts of a sound, or even different parts of a sound that must be captured independently.
A good example of this is how I record Sanshin, which sounds sort of like a fretless banjo made of snake instead of paper played with rhythmic syncopated notes rather than arpeggios.
When I walked around and listened while the Rinken Band played, I noticed a spot where it sounded rich, and that within that spot I could easily hear both an attack and a throaty twang. To capture that I used a condenser for the highs and an old ribbon for the throaty twang, both in the sweet spot I prefered.
Rinken told me nobody had every captured the real sound of the Sanshin before. Had I not listened first and in doing so learned what was important to capture I would have ended up with something typical (thin) rather than strong.
In general, you are best off moving your head around the area of an instrument (including above and below, close and far), then placing the microphone where your ear hears the best sound.
Start with suggested positions, but put your head there and listen before you automatically put a mic there and assume it is the best starting placement.
The key to mic placement is understanding what you are tying to capture, choosing the right mic and finding the location and positioning to most strongly capture the sound source.
You may have to make sacrifices for the performance (moving the acoustic guitar mic because the musician is wildly throwing his picking arm around) or sacrifices due to available microphones (etc) but you will always capture the music if you mic with your ears instead of your eyes.
Bruce A. Miller is an acclaimed recording engineer who operates an independent recording studio and the BAM Audio School website.