I am not sure what you hope to achieve by signing all the SPs with a certificate. I have only ever signed t-sql SPs when ownership chaining is not going to work or server level permissions are required.
Maybe you are selling an application in which case you could sign the SPs with a certificate to detect alterations by customers.
The following paper, by Eland Sommarskog, is worth reading as it contains a lot of information on how to handle certificates.
Packaging Permissions in Stored Procedures