More to the point - while signing certs are part of the SAML spec (and your IdP can choose to use a different one for each SP), looking at attributes in the Ass

More to the point - while signing certs are part of the SAML spec (and your IdP can choose to use a different one for each SP), looking at attributes in the Assertion and making decisions based off of them is one-off for every SP - a few do this, but they each do it differently than the last.