We had a similar rule - the 3 bite rule- with the kids. It worked fairly well. and my kiddos who wouldn't eat mushrooms or blue cheese or shrimp are now cured - a more grown up palate has developed. (They can pay for it now from their own paychecks
And after college biology class with Doc Roberts, I would NEVER eat any sushi except maybe a vegetarian combo, because of the parasites that are found in raw fish. I just don't need that kind of trouble in my life!!