You can set the rounding for billable time entries using four rules:
No Rounding (default)
Round to nearest
Round up to
Round down to
The last three can be set in intervals of 6, 15, or 30 minutes and the setting will only affect billable time, not worked time.
For example, if you select round up to 15 minutes, if a worked time entry is 13 minutes long, the billable time entry will then be 15 minutes. Similarly, if you select round down to 6 minutes, a worked time entry of 14 minutes will automatically become 12 minutes.
To set it up, go to Settings > Time tracking and scroll to Time Rounding: