Registration and ownership.
THE RULE
Register client domains in the client's name, with their details as registrant.
Never in yours.
WHY
The registrant record determines ownership. A domain in your name that the client believes is theirs is a dispute waiting to happen.
It also makes you responsible for something you should not be.
IF YOU MANAGE THE REGISTRATION
That is fine. Managing is different from owning.
Register it with their details, and hold the account access as their agent, with that stated in writing.
RENEWALS
Track them. A lapsed client domain takes down their site and their email, and they will blame you regardless of whose responsibility it was.
Set auto-renewal where you can, and remind them well in advance.
WHEN THE RELATIONSHIP ENDS
Transfer the domain to them, promptly and without obstruction.
Holding a client's domain as leverage is the fastest way to destroy your reputation in a small market.
WHAT TO PUT IN WRITING
Who owns the domain Who renews it What happens when the relationship ends