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.]