I do keep a short note/description next to the hours (when I do a consulting job). Rarely but it's happened that I had to justify the hours, and it happened 30-60 days after the bill. Without the notes, I would be lost. :) I started this habit after a book or blog on IT consulting that I'd read, but I can't remember now where it was.
I suppose I could just write "Programming" beside each entry in my billable hours log. :)
Most of my work is backed up by comments on issues, chat history, checkins, or other work-product. Though I suppose the moment I'd be requested to justify something I'd assume the relationship is no longer good and would seek new employment.
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