I've been recording a similar time, and for me - the number of times I record one instrument with two mics is very low. I do it on every instrument that produces sound from different locations that we hear with our ears as blended.
If you set up a stereo pair and view the result on a visual scope it's so easy to see width. Looking at an acoustic guitar from 10ft away, the display looks like the source is around 20 degrees of the full 360. For me, that's mono - a point source. I appreciate that the fingerboard tone is different from the sound hole tone, and if you want that close in, precise and clean sound, then two mics will need to be used, blended together. That is NOT the same as stereo. For years, the BBC would blend mics together to get the balance, for mono. Sure, we can put two mics on a harp and then do wonderful swirls in hard left right 'stereo', but that is an effect, not reality. A swirl from the harpist in it's place in the stereo field of the orchestra is not audible as a left/right shift.
Having two ears means you have the capability to locate in space a sound source. That's it. A guitar in a room is a point source. Not a point source that a trumpet would be, and different to a piano. We're talking about physical width. That's a physical thing. Stereo is in use the other end - any the receiver.
If we wish to record realism, then even with close miking, we need to be careful with panning multiple mics apart. In my head, this means that in the studio when mixing, I close my eyes and use the two pan controls to place the two miked guitar in the right place. Opening my eyes usually reveals they are in virtually the same place - a small difference - never for me, one left and one right, because that creates a guitar 8 ft wide - the distance between my monitors. Context is everything. I tend to record guitars, saxes, flutes, clarinets (because we use them all the time) with one mic - moving it to get the balance right. Drum kits and grand piano have more than one. With both these again the range of the pans is quite limited. Phil Collins might use exaggerated panning on a few tracks for effect, but for the majority of the time, drums are central in most recordings.
I do understand what Jimmy says about the room, though - and I can see that for perhaps recording a classical guitar in a church, an X/Y pair could be really nice - in the studio I wouldn't bother. Ten years or so ago I did a couple of projects with a well known classical guitarist - who cared so much about his tone that his guitar had no varnish or finishing, because he liked the sound and refused to have it finished. He gave me a single, well used AKG 451 microphone. Told me exactly where it had to go, and that is what I recorded. No input from me whatsoever. He was spot on - a wonderful sound. I'd have loved to experiment, but that wasn't going to happen.