Tipping, while always appreciated, is not required.
When a job is completed you will see a screen that will allow to rate, tip the valet and leave a comment.
If you do not see the tip/rate screen or you accidentally close out of it, you still have the option to rate and tip by going into the app menu, clicking on billing history and selecting recent transactions. This will allow you to rate any jobs that have not been rated or leave a tip on any that have not been tipped. Once a job has been tipped/rated it is not possible to change the amount.