Should I report a 48th percentile math GRE subject exam score for programs where it's optional?
Since the admissions office receive applications in large numbers they would prefer to work the application that corresponds to the "required list", otherwise it would delay the process. Sending in documents, in your case a low subject score, could have an adverse effect on your application. If the application does not require a subject score, do not send a weak subject score as it will not make your application stronger.

There is a GRE general test and subject test. As far as i know, the general test is mandatory for most programs, but the subject test is optional. 

General rule: do not send anything that is optional and does not strengthen your application.