I usually don't have much more orchestration going than some strings (and piano, if piano falls under this category), so panning isn't something I've thought much about.
However, I've been thinking a lot about ambience lately and what role it plays in different types of musical arrangements. For instance, I've found that there's a big difference between adding an orchestral section to a full-blown metal mix sparse mix that's mostly comprised of some piano and vocals. The samples that I use are recorded from two perspectives, one which is up close to the instrument, the other which is from the actual stage (it's more of an ambient, roomy sound).
I've made a couple of demos which feature a rock band type of arrangement with a string section, and for this the second perspective (stage) just fit perfectly in the mix right away. Then, not too long ago, I was working on a different song, pretty much comprised only of a piano at that point. So, I reach for the stage-perspective string section and find that it just doesn't seem to cut it. It sounds too distant. So, I switch it to the close perspective, but that isn't right either.
Eventually, I found that a blend of the two does the trick. I also realized that I can do the same trick with the piano (it comes in multiple perspectives), though with a little more control. During the verse, I'll pretty much only have the close perspective on, but when the chorus kicks in, I'll ride up the Piano Room fader, and suddenly it's like a blanket is being lifted from the instrument. It opens it up and gives this section of the song a much-needed impact.
I've found that a lot of mixes that sound interesting to me, have this balance between elements that are ambient (i.e. generate reverb) and elements that are mostly dry. It seems to build a kind of contrast, if that makes any sense. I may be way off, though. It's happened now and then that I've held opinions about music production that turned out to be very bad ideas, but it's all a learning process, right?