read only user

when creating a read only user, is it better to grant the select grant to a user or to create a role and grant the grants to the role then give this role to the read only user.

In the case of EBS.  Should you grant select on the ebs-schema.table such as ar.table or po.table or to apps.table?

My dba granted select to the tables ebs-schema.table.  This was ok, then the user complained because they use apps.tablename and so he granted apps.table to the read-only role.

When he did this, 5000 objects became invalid and everything stopped.  What would cause this behavior?