Good clients will occasionally have an oh-cripes moment and need an hour from you in the middle of the day. Good clients in an oh-cripes moment will think your day rate is a perfectly reasonable price to pay to get out of an oh-cripes moment. [Edit: On reflection, my actual practice for this is exactly equivalent to Thomas' as described in his comment. Do that.]
Why does this work? Because I don't bill anyone for an "hour or two of work", and don't have to worry about this level of accounting.
Clients notice. More importantly: clients are much more willing to reach out and ask questions when they know the meter isn't going to be ticking. Matasano has never employed a salesperson; we get our business from repeat customers and word of mouth, in large part because we are very easy to work with.
See what I mean about good practices emerging organically from not billing hourly?
Just billing a full day once I determine what they need isn't trivial would have a nice effect on both my guilt and bitterness in these situations.