without a testimonial, just like:
Our clients: Logo 1. Logo 2. Logo 3
Depending on your agreements contracts with your clients you may not strictly have to but in general it's considered a best practice to do it. Some clients may not want to be listed, not because they are unhappy with your product, but simply because they choose to not be listed anywhere. Other clients maybe happy to be listed. You should always ask otherwise you risk losing the relationship.
White Hat Answer- No, protect your clients in case your services or software could be used as a gateway into your high profile clients.