Do you routinely perform muscle biopsies in patients presenting with the classic symptoms of Inclusion Body Myositis along with positive CN1A antibody?  

YES. There are new consensus guidelines from ENMC about the diagnosis of IBM (Lilleker et al., PMID 38522330). They define the clinical presentation of IBM as typical (age >45, progression over >12 months, long finger flexor weakness > deltoid weakness, quadriceps weakness > hip flexors)...