If you have multi track mixing facilities, you might want to put a mic on each side of an accordion. Many recording engineers routinely do this, and it's certainly the best way for live sound to avoid feedback, but I've also seen a very respected and capable recording engineer put a single mic in front of my melodeon, 2-3 feet away pointing at the middle of the bellows and picking up sound from both sides. I think the mic was a Neumann M147, and it sounded fantastic.

For the solo melodeon track I contributed to the EFDSS Hard Core English Album, I used an XY pair of Schoeps MK41 directly in front of me and 3-4ft away. My studio is a conservatory generously lined with bass traps, mostly slabs of 100m thick mineral wool, so the room sound, while not dead, is quite well controlled.

I like the sound of an EV RE20 close to the floor on my cello.
There's plenty of material on the web about mic positions for other instruments, but usually with woodwind, whistles and bagpipe chanters it's a mistake to point at the end of the pipe as the sound comes out of the finger holes. Brass is a different matter...

By the way if you're interested in acoustic treatment in the UK, my favourite material is the Acoustic mineral wool from Custom Audio Designs. wrap each slab up in a cloth cover (mine were used curtain material stitched up with a sewing machine) and ideally hung on a bracket to hold it spaced 100mm away from the wall, or suspended from the ceiling with a similar gap above. The material seems to be a good substitute for the Owens Corning 703 glass fibre that is popular for the same purpose in the USA. It's not hard to make such treatment temporarily installable in a room normally used for other purposes, as long as you have space (shed/garage/loft) to store them when not in use.

Anahata