I just saw Black Swan the other day and I was impressed by the sound work. One thing I remember was in the scenes where they are dancing in the ballet studio. There was a pianist accompanying them in the room as they rehearsed. The camera tightly follows Portman as she spins dancing around. The piano (which is in mono) is panned to match the movement of her head, so we hear what a dancer would hear during rehearsals. It's great POV sound work and really gets us in the characters head, but it is also distracting in a good way. There's great use of this POV panning in the Hurt Locker and Children of Men to name other examples.
A mono rain against window hard panned to match camera perspective would be really distracting unless it is serving a story point. The idea of using rain recorded at different angles and cutting between them on every shot also sounds distracting to me. I am convinced that the human ear is hyper sensitive to changes in the stereo/surround field; which is probably a survivalist trait which has kept us from getting hit by a car as pedestrians more than a few times in our lives.
If I were looking to create a non-distracting rain on window ambience I would consider a few things:
1) Start with your rain on window panned to match the camera perspective, ideally on an establishing shot which tells us we're in a house and it's raining. Then move it to the center on the next shot and keep it there. Your panned rain serves its purpose of establishing a sense of place and then gets out of the way. I doubt your audience (with the exception of a few sound nerds) will notice that you've given up on perspective after the first shot. I've seen some seasoned sound editors ignore perspective when cutting backgrounds, especially on scenes with rapid picture cuts.
2) Even though the rainy widow might be on the left in the first shot, think about the sound of interior rain. You most often hear it all around you inside. The sound of rain off of the roof, sliding down exterior walls, or slapping off of a surface away from the window. Create a stereo/LCR/quad/surround ambience that matches this feeling, but doesn't pull the ear too far to one speaker. Pan one stereo window recording of rain to the front speakers, and perhaps a more distant rain interior to the surrounds. That way you convey the depth of the space without a bunch of panning. Once you have a nice enveloping ambience, you'll find that you can use additional mono rain sounds (with perhaps a touch of stereo verb or slap) panned here and there to spice it up a bit,
Just my 2 cents. Good luck.