Many pesto recipes call for toasting the nuts, and letting them cool, before adding them to the blender. Those recipes say it enhances the flavor. Personally, I don't do it because I don't think it makes that big of a difference. I've done this kale pesto with walnuts that were toasted and with raw nuts, and other pesto recipes with toasted and raw nuts, and the difference is so small. But -- and this is a big but -- nuts burn quickly. It's really easy to overtoast them. And then you end up with a burnt flavor in your pesto or you end up throwing those away (and nuts are too expensive to waste). So, you can toast the nuts if you want to, but you don't have to.