If all the elements of your application are rock-solid and you're worried that your score might weaken your overall packet , I wouldn't bother sending in the subject scores unless they were stellar (85th percentile and above). In any case, some of the programs for which the subject exam is optional have explicitly said that, while a high score may give you a boost in the selection process, an average or low score will not hurt your application (UPenn comes to mind).
Also, when I ordered my ASRs a few days ago, I was able to send in my general and subject scores on the same report without having to pay an extra $23 for the subject exam. You might want to look into that, too.