When is it most appropriate to post a preprint? I am a student who is very enthusiastic about open access and sharing research. When I worked with my former supervisor, we would write a paper and then post it to a preprint server (e.g., arXiv) at the same time as submitting it to a venue. My current supervisor, however, is of the mindset that we should post a preprint after receiving confirmation that the paper was accepted to some venue.

Is there a generally-accepted time to post a preprint?
It makes the most sense to put it on arXiv when the preprint is complete (i.e. when xou submit it to a journal). (Some people even post to arXiv, wait a few days to maybe get interesting comments from other people (comments which improve the paper), and then submit it to a journal.) So the community can use your results before formal publication.

However, if for some reason or another, your supervisor does not want to do this, then their opinion trumps this argument.