For enterprise contracts, it's generally a good idea to ask. You should have a clause in your contracts that allow you to use the customers' logo and that they agree to participate in success stories. Some customers may want to reword it, but many will leave it in there if it is standard language. If you do get push back, you can always take it out.
For self signup websites, you can add a clause to your terms of use or privacy policy so that you can use their logo.