As a software engineer at a fairly large printing house, I'd have to say that QUALITY prints will need a better dpi than 72. 300 being about the minimum. Most digital printers (I assume your not making 10000 copies of your calendar) eat wasily 1200 dpi images - the quality is directly influenced by your choices in making the rendering. Most home-users will have limited amount of memory though, so maybe the suggested 300 dpi is enough, but in that case you should run a production-quality test -run before you submit your work to a printer - in case you are using a commercial printer.