If you are adding apples and they will be mixed into the batter you don't need lemon juice. The apples won't get brown since they will be baked. If you are adding raw apples to a baked cake as garnish or decoration then you should dip them in some lemon juice to prevent discoloration. Also if your apples are tart enough for the recipe, such as Granny Smiths, then you don't need lemon juice. If you are using sweeter apple like a Macintosh and you want a tarter flavor, add a few teaspoons of lemon juice to the recipe.