Hey jrp...I just started making mj butter this year, so I'm no expert by any stretch, and I usually make a new batch at the end of a grow. During the grow and into flowering, I begin to save all my clippings, dead or dried leaves, trimmings etc. in a wicker basket I keep for that purpose. I definately save all the leaves from harvested plants, then finally add all the bud trimmings.
At the end of a grow (and I'm just a little guy with a little room) I usually end up with a decent pile of leaves to throw into the pot.
I just figure why not toss them all in. If they have a load of THC or just a little, if you boil and simmer long enough, and enough times (I boil and simmer at least four times over three days, letting the pot cool completely each time), you'll squeeze every atom of thc out of that mess.
What method do you use for making butter? Not the best area for this discussion probably, but hey....
-Saul-