As described in the instructions, I always recommend adding the jalapeno pepper incrementally the first few times you make this sauce. This gives you a good idea of your preferred heat level. If you are sensitive to spicy foods you can start by adding ¼ of the jalapeno at first and tasting for heat level.
If you are using canned tomatillos, don't bother roasting them. It doesn't have the same effect on tomatillos that have been resting in liquid.
Using the cilantro stems is common in Mexico. I typically twist off the thicker, bottom portion of the stems but keep the upper portion of the stem that connects the leaves.