We share all the time. My DDs are "adults" in disney's eyes, but can't eat a full adult meal. So we usually share 2 meals between the 4 of us. DH sometimes gets an appetizer too if he is hungrier than normal or the portions are too small (never an issue at Disney). We obviously do not share at the buffets or dinner shows, etc. But since the kids have become adults, we have minimized our trips to those and try to go to the "order off the menu" type places so that we can share. Its never been a problem and they usually split the entrees in the kitchen and bring each person half on a plate.
Missy,
Of the places you listed, we have shared at Teppanyaki and Sci Fi. Teppanyaki was before the dining plan. I have heard they are more strict now, but can't say from personal experience. When we shared there, the chef piled veggies, rice etc. on each of my and DD's plate and then just split the meat between us. At Sci Fi, DH and I shared the flat iron steak sandwich. We didn't share at LeCellier, but that was because we were on the Candlelight Processional package. But I was basically full after the cheddar cheese soup and the breadsticks! So we definitely could have shared there.
Maggie