Thanks Andrew. That's some pretty strong opinion on either side of the fence.
I ended up changing the mailnow string to be Send mail quicker (disables 30 minute editing period) and explained that emails go out hourly in our howto video I put together on Friday (we do a new one every week, and having had questions on it from a keen tutor at the start of the week it made sense to respond to that).
Our cron takes 5 minutes, when we timed it. It seems to error on a handful of courses, when we get the email report, but I've not looked at if there are error reports for this.