I'm looking for a new apartment, and I'm wondering if my monitors/desk should be placed at the wider wall of a rectangle-shaped room, or the narrower wall. What's the common rule of thumb for this? It would make sense to have them at the narrower end, so that the reflections take longer to travel back the length of the rectangle?
It's a bit of a tradeoff. Place the monitors on the narrow wall and you get better bass response (longer path fore and aft), but you also get more polution to your image and mids from the side bounce. On the other wall you get less immediate reflection, but won't be able to handle the lower frequencies as well.
For orchestra you might choose the better image. For hip-hop, go with the bass.
In either case the top priority would be to treat the walls that are the closest together. In the one case you want to damp/disperse the side reflections at the point where you would see the speakers if looking in a mirror. In the other case, treat the back wall to absorb low frequencies. Or do both!
BTW, bookcases with the books at odd positions, and some room behind them are good for dispersion and bass absorbtion.