I'd never ask someone *after* a meal if they didn't like something that they left behind, but I will ask when inviting them if they have any dietary restrictions, allergies, or anything in particular that they don't like. FWIW, though, if someone asked me the same I'd tell them that I avoid shellfish (makes me gag, then serious digestive issues--not a real allergy, but definitely an intolerance) but wouldn't mention any non-main-dishy things (that is, I also can't stand mushrooms, but it's rare that a meal is based on mushrooms, so I've just gotten adept at picking them out). As a fairly picky eater myself, I have a great deal of sympathy for guests who are served a food that they don't like and are then faced with gagging it down or just leaving it behind. I wouldn't dream of being insulted if someone left a food behind. So they didn't like it. No big deal. I also tend to serve meals to guests family style so that they can serve themselves as much or as little as they like of a food, or do some careful editing of a meal to make sure not to pull out too many of whatever it is they might not like.

The only exception I'd make would be if someone was a house guest and I'd have to keep feeding them for several more meals--in that case, to be a good host, I'd want to avoid a food they might not have liked again. After one meal at a person's house, the guest can go home and gorge on anything they like. But a houseguest might get hungry if they don't like peppers and I served stir fried peppers with dinner, peppers in the morning omlette, and steak sandwiches with fried peppers for lunch.

Sarah