I think it's a good idea to track your time even if you don't have to submit it to your client or you charge a flat rate, because at the end of the month (or whatever interval you want) you can go back and compare your hours to your income and make sure you are charging the correct amount. You may think you are making $X per hour but at the end of the month when you go back and compare the hours actually spent vs. the rates charged you might find out it is more or less than you expected.

When I invoice in QuickBooks I have my retainer and per project fees categorized separate from my hourly income so I can easily compare it against my time spent and see if I am over/under charging.

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Rebecka Melson ~ Virtual Business Services