Is there going to be an increased the load on the server to enable 1 or both RBL's listed in Exim configuration in WHM? maybe because of excess scanning done by the server, or would it actually lower the server load - cpu/mem usaged because the server is receiving less spam?

Is it wise to enable these RBL's on a shared hosting environment server, or should it be left to the users themselves to configure spamassasin on their own?