After encountering a few official forms to fill out (tax, medical, LinkedIn, job applications), I don't know what my official job title is during my PhD. There doesn't seem to be a standard for this. I don't think "student" accurately reflects my role, because I'm doing research and getting a small stipend for it. What should I put down as my occupation?
There's really three different things mixed up here.
Your job title, as an employee, is whatever it says on your paychecks. Maybe "Teaching Assistant" or "Research Assistant" or something similar. This is what you should list on job applications, etc, under "employment". No choice here.
Your educational status is "graduate student", "PhD student", or the like. You could also use language like "PhD (in progress)" or "PhD (expected completion 20xx)". You might be a "PhD candidate" but defer to your institution's rules as to if and when they consider you to have that status.
Your occupation is just a word to describe the field you are in and the type of work you do. So you could list "biologist", "historian", "physicist", etc, as appropriate. You could also choose something more generic like "researcher", "educator", "scientist".
For tax forms in particular, I don't think it makes a lot of difference how you describe your occupation. I don't actually know how they use this information (maybe a good question for Money.SE), but I've always assumed it was mainly that if you gave your occupation as "oil baron" or "railroad tycoon" and then reported a tiny income, they'd probably audit you.
During the time I was studying for my PhD, I put "Student" as my occupation on my income tax forms, even when I was being paid as a TA or GSR. That was after my industry career, so I had a more complicated financial situation. The returns were prepared by a tax-specialist CPA who agreed with "Student".
I had a lesser form of the "oil baron" issue Nate Eldredge mentioned: the change in occupation from "Computer Architect" to "Student" explained both the disappearance of the relatively high wages I had been reporting, and the appearance of educational expenses.
Since you mention title for tax purposes and similar, if I were you I would assume the title of whatever your "job" functions are. Generally, in the case of a PhD candidate, you are usually either a "Assistant Lecturer" or "Doctoral Candidate Research Assistant." Otherwise, you are a Doctoral Candidate, which I would use to explain my job title, rather than student.