Many sources add lemon juice to fruit butter. My Ball Blue book doesn't call for it. And other online extension service websites do not call for it. Lemon juice would add to the flavor and likely help maintain a nice color so it certainly won't hurt, but it is not required. Most times you must use bottled lemon juice for canning recipes as it has a standard acidity level. In this case it is more for flavor... so fresh lemon juice would work if you've got lemons available.