It's subjective -- whatever you like can be. However, if you're not going with one of those big orchestra layouts in the above pictures, I suggest putting the bassy instruments in the middle of the stereo spectrum to ensure that the bass power is distributed across both speakers/earphones in a home listening environment, even though most graphs have the basses and tuba on the right. It sounds more balanced.
And I wouldn't pan the strings too wide unless you want every little detail to be listened to, because it can sound cartoonish. Maybe -10 for the violins and +10 for the cellos, with violas in the centre. But if the strings are playing in unison most of the time, you can get away with panning them wider and it will give it fullness.
I would distribute the woodwinds and brass to the left and right as you please, such that when everything is playing, the left and right channels are equally engaged. Again, no really extreme panning unless it's for a certain effect.
To differentiate between closer and farther instruments, use less (closer) or more (farther) reverb. But don't overdo it.
That's just my ideas on the topic, someone else will probably disagree.