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.
If there's an NDA, it's a strict no for sure...but there are times where it's better to show the landing page, maybe the way it will be showcased and then ask if they have a problem with this... or if this needs to be taken down.
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.