If one eats before a meal in a number of cases one is exempt from making a Bracha Achrona because it is exempted by the Birkat HaMazon after the meal. In other cases, though, the foods are not exempted and require a Bracha Achrona as usual. Lastly, in the cases where there is a dispute as regards to the halacha, one should be strict to avoid that situation. See further for details.
# If a person drinks wine before a meal one should not make a Al Hagefen afterwards. However, because there is a dispute whether the wine is exempted by the Birkat HaMazon it is proper to avoid this situation by either by drinking a melo lugmav and not a reviyit, having intent when saying Birkat HaMazon that it should cover the wine, or have intent that one's Hagefen upon the wine before the meal should exempt the wine that one drinks in the middle of the middle as long as one does have wine in middle of the meal. <Ref>Vezot HaBracha (chap 9, p. 84) writes that because drinking wine before a meal involves a dispute one shouldn't make a brahca achrona. However, he quotes three advises from the poskim to avoid this situation. The first is to drink less than a reviyit in order not to become obligated in a bracha achrona. The next is to have in mind when making the Birkat Hamazon that it should exempt the wine one drank before the meal. Lastly, he advises that if one will drink wine in the middle of the meal that one has intent that the hagefen before the meal will exempt the wine in the meal. </ref># Regarding Kiddush, according to Ashkenazim it's preferable to drink a Melo Lugmav and not a Reviyit.<ref>Biur Halacha 174:6 D"H Vechen, Vezot HaBracha (chap 8, p 84)</ref> According to Sephardim, it's preferable to drink a Reviyit and have intent that the Birkat HaMazon should exempt the wine.<ref>Or Letzion (vol 2, 20:22)</ref>