I am a web designer registering a domain name for a customer – who should be the “registrant”?

You should register the domain against the “end user” company – if a company is paying for you to register the name it would normally be registered under their name. Transferring domains at a later date is a time consuming process and may incur additional costs. You should also make them aware of their obligations – the registrant is the “legal entity” bound by the Terms and Conditions and Domain Name Registration Agreements.