Twill beat me to it; you want to prevent the growth of algae with zinc strips run jus tunder the ridge cap (they are usually 2" wide x 50' long in one long run that can easily be snipped to length).
If you have a hip roof, the mfg's instructions say to put a 6" long piece about every few feet, but to me this would look kind of odd...
Another thing is that sthis should go on both sides of the ridge or hip cap if you want "true" protection from algae. One of the problems with algae is that it can prematurely age the shingles & over time, it really gets down into the base causing grit loss.
Here's a suggestion - get a package wrapper from some of these 50 year shingles, or @ least the mfg's brochure & see exactly what they list as exclusions to their warranty. Also, note where there is a sliding or pro rated scale of what they pay because even a 50 yr. product won't have 100% coverage.
Finally, your primary concern is to never have to re-roof again so don't look ONLY @ the shingles. Consider what can be done for the best & longest lasting form of:
-- Ventilation. Improper air flow = shortened shingle life.
-- Ridge cap if you have steep slopes & hip cap (as I like to tell my customers, most 30 year roofs only have 20 year seams)
-- Flashing, counterflashing, valley, etc. Why have a 'perfect' field but lousy termination joints? If you wanted to go all the way, you could always have the entire permiter & along valleys with some kind of a peel & stick product or other cold weather applications (I'm in Texas & we don't usually do much for this kind of issue here in the Hill Country).
-- Same goes for chimney flashings or any concrete to metal connections.
Also, pay attention to color - make sure it's something you'll like in 30 years!