Hey!
Well, I do not actually work for an agency but for an IT company. 
Regardless, we faced similar decisions and, like everything in life, the answer is: it depends. How many clients do you have and how many tasks are there per client? Also, how long do you usually have the client actively being worked on in Asana?
We have made the experience that the way you use Asana should be based on how easy it is for you to get a quick overview of your work at all times. So, for example, if you have lots of clients, but each of them only hosts a few tasks at a time, it might be too scattered to use a team per client. If you, like us, have very few individual clients and lots of tasks per client, it makes sense to have a team per client.
I’d be happy to provide further insight if you give me some more details about your requirements. Maybe it even makes sense to not divide your setup by client, but to actually form teams, such as SEO, social, etc.?
Best,
Tom