Much like your billing rate, your billing increment is not a discussion, it is an announcement. Most of your customers are going to agree to it automatically, and the ones who don't are giving you fair notice that they're going to call you and need precisely 89 minutes of your time ("rounds to 1 hour, right?").
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.]
Worth noting: good clients tend not to pay anything for the 1-hour "oh cripes" conversation. I'm extremely unlikely to bill for an hour or two of work.
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?
This really struck a chord with me. I always feel really awkward billing for an hour or two - either because it was no sweat to help them out because I wasn't doing anything anyway, and it was barely any work (a phone discussion and changing a couple of lines of code), OR because dropping everything to put out their fire pretty much hosed most of my day.
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.
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.]