As a little followup to these great responses, I bill in different ways. For example, for straight text-to-text typing, proofreading/editing, and translations (also data entry could be in with this group), it is a flat fee -- depending on what it is. Transcription is simple: per audio minute.
For hourly clients, I also bill in 15-minute increments. I learned this from working in a temp agency -- we bill our time in 15-minute increments. I guess I carried that with me.
I know for me, these invoicing rules are subject to change. Usually I ask the hourly or retainer client if they prefer a breakdown on their invoice. I have yet to have one tell me they need a detailed list of what I did.
For any a-la-carte or transcription invoicing, I always include details as far as document/file that was worked on. I think it helps the client sort out their own paperwork.