I tag via the pip-boy mostly. Choose the junk directory in the pip-boy, choose the shortkey to see what components they break into (think it is C) When the components are listed, you can click on them, and choose Q shortkey for tagging/untagging. The limiting factor is of course, that you need to have a junk item that breaks into what you want to tag. Haven't been any troubles for me tho.
wow I have a ton of hours into this game and was wondering the same thing. I had no idea. thanks!