I always line the bottoms with baking parchment. Upside down cakes don't have enough fruit on the sides to make me worry about a small chance of the acid reacting with the pan. Lemon cake has little lemon juice (most of the flavour comes from lemon oil or extract or peel).
The chemists on Cake Central will all assure you that "aluminum" pans have a natural microscopic coating of aluminum oxide (same stuff as rubies and sapphires) which is invisible but pretty tough. I have 30 year old aluminum baking pans that have no sign of being eaten away on the inside--they get dents if you are careless, but there is no longterm sign of any interaction with the food.