The majority of off-the-shelf rooting compounds have Indole-3-Butyric Acid (IBA) as the active ingredient.

And they all say not to use it on edible plants.

However, I'd swear that I read somewhere that the table grape growers use the stuff to spray their grapes to get them up to the size of those enormous, tasteless ones in the grocery stores - but I can't anything right now online.

But personally, using the minute amounts of the chemical that you'd use on a cutting, and then later on eating a lemon from the cloned tree, I wouldn't worry about it. I've used it to clone pepper plants and French Tarragon.

You could always try using willow water - simply cut a handful of willow stems into 2 inch lengths (any will do but the shrubby osiers such as viminalis, S.purpurea or S.daphnoides are easy), soak for 24 hours in water and dip the cuttings in the liquid - it contains salicylic acid - one of the active ingredients in aspirin.

I like Olivias - not the cheapest but, if stored in a fridge, it will last for months and months.