The server need to have certificates on them, however they do not need to be same certificates that you use in the SSL client profile.
When you do SSL offload and you have and you want to use SSL between the F5 and the servers there are 2 SSL connections:
END USER <--- SSL1 ----> F5 <--- SSL2 ---> Web Server
Between the End User and the F5 the F5 will use the SSL client profile. Here you should use a certificate that is signed by a well known CA.
Between the F5 and the Web server you can use the same certificate, or you could use a self signed certificate. No matter what you do the Web server will need the private and public part of the certificate installed on it. One advantage of using a self signed certificate is that you can make the expiration date as far out in the future as you want and thus you don't need to replace/update the certificate that often if ever.