No - account registration is optional.
However, registering for a free account gives you a host of benefits, including the ability to:
Upload multiple files at the same time
Merge documents - bring together multiple files into one printed document
Store & organise files in your secure file library
Save product settings
View your order history
Option to reorder
Plus, after registration you will be emailed a 10% discount code to use on your first 2 orders!
Happy printing!