Just to be on the safe side, I check the Nutrition Information on something and add it - even though I may have added it before. I noticed, for example, that over the years, McD's has changed the calorie count on the Filet-o-fish; there are fewer calories now. The reason, I think, is that the size has changed - it is slightly smaller.
It's just my habit. I think I would do whenever I use a food database.