If the program you are applying to does not require (or only suggests) the subject test, the rule of thumb as I have learned it is: DO NOT TAKE IT. SOme majors (mathematics, for one) will find that most programs require the subject test and appear to lean heavily on it. I have a friend (def not me, I'm in counseling) who is applying to PhD programs in math. His general GREs are V167, Q164, Essay 6.0, has a 4.0 GPA, is Phi Beta, take several math indecent studies, heads the tutoring program at his school, and likely valedictorian at his undergrad. He also had faculty falling all over each other to write recommendations.
Well, he had a very bad day at the subject test and got a 580 (33 percentile). He's heard from 5 programs so far. 3 required the subject test and the results were reject, reject, wait list. One 'suggested' the subject test: accepted with some funding. The last did not require the subject test and the result? Accepted as a fully funded TA-stipend, tuition, health benefits.
Now, it is true that the GRE Math subject test is considered by many to be one of the toughest standardized tests out there (as a result of too many perfect scores on the previous iteration about 8-10 years ago), but it's clear by my friend's results that if you have all the other requirements in line, you can only hurt yourself if the subject test is not required. If you feel your other numbers are on the bubble, you might have to sit for the exam.
Good luck!
