My tendency is that if I enjoy the story, I purchase it.
My rule is if I finish a volume I buy it, even if I wasn't thrilled by it. I figure if I made it that far then I should support JNC licensing it.
That's easy for me since I have a well-paying job and my hobbies are fairly cheap. It's not something I'd tell anyone they're "supposed to" do.
If you end up blind-buying later volumes, be prepared for my warning above about how the... pace... slows... down......... in........ later............ volumes. Kind of like how Unwanted Undead Adventurer spent one volume just getting ready to go on a trip.