I think lilies are one of those flowers that are a bit of a false friend, so to speak, because they have certain distinctive ‘lily-shapes’ that you can draw a convincing enough lily with, without really having to look at the thing in front of you.They’re too easy to ‘conventionalize’, I mean. But I guess it depends on what one wants from a drawing. If it’s about just looking, then that’s an extra hurdle. if one wants a nice drawing it might help. (I generally rationalize this choice after I’ve done the drawing, thus cheating!)