Here's a few rules I usually think about with respect to planning:
We can't determine the direction of low frequencies, so the bass is often placed at the center of the mix. Panning the bass hard to either side would especially be noticeable by headphone listeners, and potentially very distracting unless that's the effect you're going for.
If you're producing for a general audience, then there will invariably be listeners who are set up with only a speaker on the left or right channel, or where the listener is much closer to one of the speakers. To produce for the broadest possible set of listeners, you should to pan all the really essential parts of the music close to the center -- lead vocals and instruments, as well as basic rhythm tracks.
Panning almost identical takes hard left and right creates a chorus like effect.
Other than that, I use panning as the simplest, basic way of balancing the sound (duh :-). For example, if I have two guitars that overlap a lot, then placing each further to the left and right will help separate the guitars for listeners with a proper stereo sound, while other listeners will not loose anything essential to the music.
From there on, it's usually just trial and error for me. I take each track and ask myself if it is essential and/or if it overlaps with other tracks. Do what sounds good, as you say.
I would probably try to center pads/strings that only play slow and discrete sweeping parts. But if they play a more active role, then they might balance the acoustic guitar so panning the strings and guitar respectively left and right might be an option. Some synth pad sounds already come with a wide stereo sound, so that will have to be factored in as well.