As formally adopted in a forum discussion, there should be no more than one blank space between paragraphs, sections, and the like. Put another way, a single blank carriage return ↵ between paragraphs or sections is sufficient.
Do not put blank carriage returns ↵ at the top of a document or after a template, such as an infobox.
A single blank carriage return ↵ should separate the main body of an article and the category list. In the Monobook skin — and in Wikia, if you turn off the category module — the categories appear at the very end of the document. If, however, you're using the Wikia skin defaults, you may think that the categories are in a separate little box off to the right of the editing screen, and there's an extra carriage return at the end of the document. Do not delete this extra space. The bot is just going to put it back, anyway.