Anything that needs to be in the center should go to the center, and there really isn't anything that's NOT supposed to go there, assuming it's properly mixed. For example, 99.999% of all production dialog should be in the center channel, even if the actor is walking across the screen or 2 acrors are on opposite sides of the screen talking to each other (fortunately, that type of shot LOOKS strange as well, so you don't see it very often). Reverbs are often in the LR (and sometimes surrounds), and reverb applied to ADR to match production dialog is normally mono, and also in the center.
Group walla, or actual offstage dialog or ADR can be (and often is) panned, but it's up to you and the director to decide if it works or just sounds weird. The literal placement of sound doesn't always work so well, and it's best to err on the side of caution. Also, be very careful of panning Dia/ADR to the surrounds. You get the audience turning around in the theater, distracted, and if for whatever reason the surrounds aren't playing back properly, you could lose the line (or sound effect) completely.
Foley is locked to the action, within reason, and panned to taste. If an actor is walking across the screen, or is just at one side of the frame, you don't want to pan the Foley while the dialog is in the center. That disconnection sounds VERY strange.
Sound effects that are "focused", like a door close or even a gunshot, are often mono (although comprised of multiple elements) and panned to match the situation. Wider FX, like an explosion, car interior steadies, etc work better in stereo or LCR. The thing is, you don't want a sound effect to be unnaturally wide when played against dialog. The center channel always gives a sense of anchoring the sounds to the screen, so use it whenever possible
Hope this helps, good luck
Puget Sound, Inc.
Los Angeles, CA