To avoid any dispute, it is advisable to seek written approval from clients for the usage, whether it is the logo or the company name. It is a common practice to use logos in presentation slides, and the more prominent companies are used. Usually, it is just a sample of your clients and not the full listing. Another common practice is having moving logos on the company website for the same reason.
Having said that, you still need to have a full listing of all your clients (without the logo) as potential clients may request it.
Regards,
Autumn Jane