Since we're using lemon zest as well as the lemon juice, I recommend buying organic lemons. As a rule of thumb, try to buy organic produce when you plan to eat anything that isn't protected by a thick outer layer. I try to buy apples, berries, and greens organic, while bananas and citrus fruit is usually ok to buy conventional if I'm not using the zest. That said, if you can afford to buy all organic, go for it!