In many organizations, weekly dashboards get rolled up to monthly, quarterly, or even annual reports. This is a relatively simple exercise in addition for most metrics like page views, visits, and various other events that occur at the session or page view level.
Not all metrics are quite this simple though.
When talking about unique visitors, you must always keep in mind the time period you are currently looking at. For example, if you're looking at a week's time period, it only makes sense to look at weekly uniques. This concept is simple enough but I've previously seen many people add weekly uniques together to understand how many visitors a site had in a given month. This simply does not work. What if a user visited both in week 1 and week 3? They'd be counted multiple times. You really need to re-run your unique visitor counts for the given time period.
Calculated metrics like conversion rate are another example of items you simply can't sum together. Odds are the elements that make up the calculated metric can be summed (with the exception of visitors) so it is simply a matter of recalculating the metric on a different time dimension. In other words, you can sum visits and orders so you simply must recalculate the conversion on a monthly basis.
