Always decarb and always use lecithin(except in tinctures, lecithin is not needed). I own a cannabinoid testing kit for infused oils, butters and tinctures that has shown both to be important factors to increase potency.
Decarboxolating changes non psychoactive THCA into psychoactive Delta 9-THC by oxidizing the compound which releases a carbon atom. This can occur naturally over time but is greatly sped up by applying heat.
Lecithin acts a binder to ensure the cannabinoid compounds have more fat molecules readily available to attach to.