No, it's optional but every now and then we have trouble finding our clients during pickup and a quick text message is often better than e-mailing as you'll typically see it sooner.  The driver has limited time to wait so it can make a difference.
Please note that we will only call you in rare cases during pickup when the driver is unable to send a text or email and knows you're in the area.  We do our very best to find our clients and the easiest way to solve booking errors is fast communication.