Myself, I like to start with about 15 to 30 posts, already in place, with a month's scheduled.
I know many don't bother as much, but I like to have things ready to go, before I set the site live, more or less.
I use a lot of templates, where I add other advertisements at the bottom of many posts. Especially in my video blogs, so that I can gather up a few gravy sales, though these days they are more the meat 'n taters sales.
My reasons are to check out the theme, once it is a bit populated, and if I need to tweak it, well, not a huge task with so little in place.
Another "pre launch" routine is I set my sidebars, especially the various blogrolls I'll have in place. I like to include a separate blogrool for membership sites I promote, along with my own sites, and break them down a bit, rather than simply one long link list on the sidebar.
Many themes are also now allowing for descriptions to be shown, and that also can throw off the length of the individual page.
Customizing the site map is another thing I like to do in advance, which includes adding some advertising directly into the page.