Is the Optional Update Form only for REA deferred candidates, or can RD candidates use it as well?
No. It can be used by both REA deferred and regular applicants for any SIGNIFICANT updates or changes to your application.
@gravitas2 : was it bad that I emailed my regional admissions officer (a few weeks ago), rather than using the optional update form, about a mistake (not the biggest in the world, but not small either) in my original application?
@RunningForLife. The regional officers generally don’t like to be disturbed…but, I don’t think it will affect you either way. The best way to update application for most applicants is to use the optional update form or use the following site http://admission.stanford.edu/contact/ you can ask questions or update information or correct any blaring mistakes.
Also you can fax directly to the admissions office with important information:
@gravitas2 For sending updates about awards, do I send it to the general fax or fax for applicant credentials?
@Collegian. You can use fax for credentials number or http://admission.stanford.edu/contact/(and choose “questions regarding your submitted application”) to update any awards…and if you are anal like most students on CC…you can do both just to be on the safe side 
@gravitas : Should I be worried if my regional admissions officer hasn’t replied to my email (which I sent roughly 1 month ago)?
^^regional admissions officers are to remain “detached” and “objective” to all applicants and not to “lead” anyone. Their jobs are not to become “friends” with applicants. They are not to show their hand in any way until the decisions are released. Once a candidate is accepted will a “personal note” or response usually be generated from an admissions officer…
…that is why any updates, correspondence, questions should be directed to the admissions office…not to anyone specifically. The regional officers want to remain impartial (as humanly possible) to all applicants in their region. So, please do not read anything into not getting a response from your regional officer…
what is the last day we can fill out the optional update form?
@gravitas2 : I got a reply soon after sending my email that said that my regional adcom was out of office reviewing RD apps…i’m just wondering if my adcom is going to see the email before decisions are out?
@RunningForLife. Your adcom most likely has seen your email that you sent a month ago. The updates were all due by March 1…
@gravitas2 : Thanks! I sent my update yesterday - is sending the update ON march 1st ok?
You will be fine. If they wanted it my Feb 28th, they would use that date!
Thanks @pacnwmomof2 for the assist. 
Yes @RunningForLife it should be fine. 