I dont clean the leaves, just make sure they are collected form a clean area (no chemicals or pesticides etc) and are fully dead/dried ie. collected as/after they fall naturally from the tree and have no green left in the.
there are SOME leaves can carry toxins, many hardwood/deciduous trees are safe though , but always best to research the species of tree first to be safe. I know Oak leaves are safe and my preference along with alder cones. Locally I could also collect maple, beach or a few others that i've found are also safe , but i find they all have thinner leaves which break down very quickly, while oak have nice thick leaves that last a bit longer.
