Depending on how much water your soil retains, etc-but as a general rule-don't fertilize them heavy until the roots are formed. Go heavy on the last two and light on the first one-ie...0-5-5 for a few days or 0-10-10 even. Still, I wouldn't worry about fertilizer with cuttings or even watering for that matter unless the soil drains very fast. The faster the soil drains the faster roots will form and the less chance of rot...of course one must water more to compensate. When in doubt, follow what the plant tells you. If the leaves are wilting water, if the leaves are not wilting then don't water. If the leaves are wilting and the stem is going soft, chances are its been overwatered for several days already. Once a Brugmansia has formed a good callous one can be more generous with the water and the fertilizer. #1 cause of death among Brugmansia are people trying to baby them. Listen, don't baby.
Hope this helps,