I'm in a Molecular Biology program and hence, when I was applying, my applications were to other 'molecular' based programs. Thus, many schools wanted the Biochemistry, Cell and Molecular Biology (BCMB) GRE Subject test. Furthermore, some funding programs want GRE subject tests. HHMI, which no longer offers Pre-doctoral funding, REQUIRED the BCMB subject test.
I will also add that the GRE seems to be quite important if you are applying for any sort of extramural funding (i.e., and NSF fellowship).
Judging from what I have seen or heard, some schools are just looking for a minimum score to decide whether or not to give an applicant an interview. Again, this is just like medical school admissions: some schools have a cut-off, and if your score isn't over the cut-off, your application goes into the recycling bin and you won't hear anything.
I did quite well on the analytic portion (770, 95%) but OK on the math (670, 60%). One of recommendation writers was the kind of guy who never got anything less than a 'A' in college and did extraordinarily well on standardized tests. He made a point to tell me that, "I also mentioned that despite your poor math score, that you were an extremely bright student." Moral of the story, you can still be competitive even with a less-than-stellar GRE.
Like I said, know your strengths and communicate these things with your boss/mentor/professors. Practice taking GRE exams under timed conditions, that'll help make sure that you maximize your GRE score.
Good luck.