Is there anything I need to be aware of when registering domains on behalf of a client?
Registering domains "on behalf of clients" (in some other company's name, not the client's name) is widely viewed as a very bad practice. Too many reported cases of domains being held hostage or domains not being renewed.
Explain the domain registration process to the client - especially the part about THEM registering the dmoain for 10 years - and tell the client to protect their administrative email address like Fort Knox or the family jewels.
There may be other points of view or people with other experiences who could speak to the issue of "who should register the domain and in whose name".