While you cannot round your time entries on a individual/timer basis, you can round time that your invoice your client.
The way that you access this feature is by navigating the Time Tracking page on a project. From there, if you click the button to "Invoice Unbilled Hours", you will see a modal to create a new invoice billing your client for your time tracked.
Within this modal, you can choose to round your hours to the next minute, 15 minutes, 30 minutes or hour. This can sometimes make your timesheet billing look cleaner to clients with rounded invoice amounts.
Note: The rounding will only affect the timesheet added to the invoice, but the original time tracked will remain the same on your Time Tracking dashboard.