This is largely a matter of personal taste. If you wish to savor and enjoy the anticipation, buy them as they release. If you love the authors and can’t bear to wait a year to start reading the story, buy them as they release.
If you cannot bear cliffhangers, you may wish to wait until the entire story is complete—or at least an entire season—and buy them then.
Whatever you decide, we strongly recommend you buy them onsite to enjoy the season discount.