You can access any account inside your reseller allocation without knowing the client's password.
HOW
WHM > Account Information > List Accounts. Find the account and click the cPanel icon beside it. You are logged straight in.
WHEN TO USE IT
Diagnosing a problem the client has reported Setting up an account before handover Running AutoSSL, creating mailboxes or configuring cron on their behalf Taking a backup
WHEN TO BE CAREFUL
You have full access to the client's email, files and customer data. Treat that seriously. Access it to do the work asked of you, not out of curiosity.
If you handle personal data belonging to your clients' customers, you have obligations of your own. Have a privacy policy that reflects what you can actually see.
TELL CLIENTS
Be open that you have administrative access. Clients generally expect it, but discovering it unexpectedly damages trust.
WHAT NOT TO DO
Do not make changes to a live client site without telling them. An unexplained change that coincides with a problem will be blamed on you regardless of cause.