My main experience is with publishing papers at IEEE conferences (the kind where a conference contribution counts as full paper), so I'll stick to that for my answer, although it probably extends to IEEE journals all the same. IEEE are pretty explicit about their rules, this is an excerpt from a document called author_faq.pdf:
Can an author post his manuscript on a preprint server such as ArXiv?
Yes. The IEEE recognizes that many authors share their unpublished manuscripts on public sites. Once manuscripts have been accepted for publication by IEEE, an author is required to post an IEEE copyright notice on his preprint. Upon publication, the author must replace the preprint with either 1) the full citation to the IEEE work with Digital Object Identifiers (DOI) or a link to the paper’s abstract in IEEE Xplore, or 2) the accepted version only (not the IEEE-published version), including the IEEE copyright notice and full citation, with a link to the final, published paper in IEEE Xplore.
There is more including the definition of accepted vs. published version (published version is "with copy-editing, proofreading and formatting added by IEEE") and uploading the paper on a personal website (also allowed), but since that's not the focus of the question, I won't copy it here.
Which brings me back to the core of the question: when to upload the paper?
Actually, in the case of ArXiv, it's already in the name: ArXiv is a preprint server, as the name indicates, uploading happens before printing / publication. At what point exactly is up to the author, I usually uploaded a paper once it was accepted, others uploaded their papers when they submitted them for review.
