Some have the latest date posted you can take it in their admissions information (University of Washington does, November 15th). It can take a little over 2 weeks for ETS to post your full score (V/Q/AW) online and send your score to your designated schools, plus however much time it takes your recipient schools to actually receive and process the GRE score (could be right away, could be a couple weeks). Of course, sometimes things do get sent faster than expected, but it's best to plan for the maximum amount of red tape to be applied to the process.
So, if you have application deadlines that are due late November or later, late September should be fine. If they are any earlier than that, you may be cutting it close. I would just email the schools you are applying at and get a solid answer, all of the admission departments I have emailed with questions have responded very promptly.