Adding secondary dns to client pc in domain?

Hello,
Ive done a little research and found people who support both sides of this question, but wanted to ask here as well. Is it ok to add the isp’s dns to the secondary dns spot on the client pc’s? Or does that cause problems in a domain environment? Its a single domain controller/dns server and the first and only domain controller in this setup. If it were to go down, they would have nowhere to go for resolution. Small store with 40client pc’s. Thanks in advance for any help…