I've never tracked my unbilled hours. I love a lot of different things about being an independent contractor, so I read books, or go online for information on contracting, construction, financial management, sales, marketing, or what-ever. Lately I've spent a lot of time with estimating software for sales sheets, and marketing & sales.
If I have 5 hours installing, I'll usually spend another 5 reading, or doing something else that's work related. If I'm working 50 hours a week, I don't do it at all.
The unbilled time I'd like to be paid for is travel, shopping, phone, bidding, etc., which are in overhead. If I get the work it's paid for, if I don't it's not.