is it a good idea to include links of my scheduled blog posts into the sitemap.xml file?

I know it might bring some 404 errors for the time they are not yet available, but is it a big problem as by doing this I can save a lot of time of tedious updating my sitemap one after another post once it is published?

There is a <lastmod> to fill on each entry in the sitemap.xml file, will it tell the crawler to start indexing this page from that date?