Jimscott,
I take leaves from all my plants, all the time, whenever I feel like it (usually every 2 months). For pings, my observations are: 1) the type of leaf does not matter 2) getting the leaf bottom does not matter 3) the number of leaves on the mother plant does not really matter (I have balded some down to 2-3 leaves). My pings have carnivorous leaves year round since they experience no temperature or light changes. The smallest leaves I have taken were 4-5mm from a 1/2'' diameter debbertiana, and they all sprouted. After trying different conditions, I have found that lower light conditions and wet APS (spritz weekly or whenever it appears completely dry) works 100% of the time. The pressure from something on top of the leaf base (APS) stimulates the growth much faster than if not. Personally, I use petri dishes, since that is what I have available. If I do not get the base of the leaf, I make sure that I will spritz with water more frequently since they will be prone to dessication (make sure that these are well wedged under something). Under the lower light the new growth will be etiolated - not to worry, just keep spritzing weekly. If I lose any leaves in a cycle, they will be those half ripped ones and a result of my neglect. I keep the little plantlets under low light until I see some roots and they are at least 5mm-10mm diameter. At this time, I will pot them up and subject them to the higher light levels.