In Surftrail, I don't do categories, I do indices.
This is hair-splitting, but hey, I've got a degree in that. Categories impliy a taxonomy, a perfect tree where everything has its place.
In my blog, everything is deeply intertwingled. I use indices to collect related nodes. Thus, most notes belong to more than one index, while some aren't indexed at all.
Want to know how I do it with Tinderbox?