Hi Bob,
If your WordPress’s scheduler is working normally, and the site gets visits, then you shouldn’t need to do anything. But if the scheduler is broken, then keeping the UpdraftPlus page open whilst you back up can help – the page will try to kick the scheduler into life, it can, whilst its open (this especially helps people taking one-time backups, e.g. of client sites, who don’t have time to debug exactly why the site’s scheduler is broken – they just want a quick copy).
David