Add an option when setting up Debit/Credit Types (in Categories & Rates) to apply a minimum time to a work item or project (independent of the global timer rounding setting). Possible implementation could look like a field under "Default Hourly Rate" called "Default Timer Increment" with a checkbox "One Increment Minimum" or something like that.
I like the way the timer rounding settings work, but some of my Debit/Credit types should have an hourly minimum applied to them, ie. 15 minute or a 1 hour minimum, and others should not. I don't want to end up charging 30 minutes for a 16 minute call (if my minimum is 15 minutes), which is what the "Always Round Up" setting does. I do, however, want to charge 15 minutes for a 5 minute call, and charge 1 hour for a 20 minute appointment (different debit/credit type) and use the global or project timer rounding default for the amount over the minimum. I really think the timer rounding settings should be assigned to Debit/Credit types and not to projects, as projects can be expected to have many different debit/credit types within them.