I've been shingling roofs for over 20 years now. I have never had to go back to fix a leak. No leaks ever, that is my guarantee. I also install all shingles to manufacturer's specifications for hurricane zones. I also install standing seam and corrugated metal roofing.
If there's one place that homeowners find that leaks more than any other, it would have to be the chimney. Properly flashed, a chimney should have flashing and counter flashing. I have a large metal break for installing custom made heavy gauge metal in the color that will match your roof. The flashing will never have to be done again in your lifetime, even if your shingles are replaced in 30 years, it will still look new and can be reused.